[Improve](Variant) pick random backend as coordinator

[eldenmoon]

What problem does this PR solve?

Issue Number: close #xxx

Related PR: #xxx

Problem Summary:

Release note

None

Check List (For Author)

• Test

  • Regression test
  • Unit Test
  • Manual test (add detailed scripts or steps below)
  • No need to test or manual test. Explain why:
    • This is a refactor/code format and no logic has been changed.
    • Previous test can cover this change.
    • No code files have been changed.
    • Other reason

• Behavior changed:

  • No.
  • Yes.

• Does this need documentation?

  • No.
  • Yes.

Check List (For Reviewer who merge this PR)

• Confirm the release note
• Confirm test cases
• Confirm document
• Add branch pick label

[Thearas]

Thank you for your contribution to Apache Doris.
Don't know what should be done next? See How to process your PR.

Please clearly describe your PR:

1. What problem was fixed (it's best to include specific error reporting information). How it was fixed.
2. Which behaviors were modified. What was the previous behavior, what is it now, why was it modified, and what possible impacts might there be.
3. What features were added. Why was this function added?
4. Which code was refactored and why was this part of the code refactored?
5. Which functions were optimized and what is the difference before and after the optimization?

[csun5285]

LGTM

[github-actions]

PR approved by anyone and no changes requested.

[xiaokang]

LGTM

[github-actions]

PR approved by at least one committer and no changes requested.

[eldenmoon]

run buildall

[doris-robot]

TPC-H: Total hot run time: 41789 ms

machine: 'aliyun_ecs.c7a.8xlarge_32C64G'
scripts: https://github.com/apache/doris/tree/master/tools/tpch-tools
Tpch sf100 test result on commit bcd1c7a2499102a6679da420744c4c56f694960c, data reload: false

------ Round 1 ----------------------------------
q1	17574	8247	8110	8110
q2	2043	181	166	166
q3	10641	1171	1249	1171
q4	10570	789	842	789
q5	7644	2954	2874	2874
q6	263	154	147	147
q7	1078	621	597	597
q8	9467	1963	1987	1963
q9	7299	6972	6888	6888
q10	7059	2329	2320	2320
q11	486	260	259	259
q12	483	220	224	220
q13	17755	2900	2920	2900
q14	262	207	219	207
q15	606	492	492	492
q16	657	575	576	575
q17	1015	619	487	487
q18	7280	6598	6665	6598
q19	1366	1129	1023	1023
q20	497	188	185	185
q21	4299	3609	3504	3504
q22	377	314	315	314
Total cold run time: 108721 ms
Total hot run time: 41789 ms

----- Round 2, with runtime_filter_mode=off -----
q1	8103	8006	8060	8006
q2	351	250	237	237
q3	3168	3045	3166	3045
q4	2209	1874	1787	1787
q5	5765	5848	5862	5848
q6	252	148	140	140
q7	2347	1777	1826	1777
q8	3633	3753	3789	3753
q9	9082	9022	9064	9022
q10	3754	3760	3696	3696
q11	626	514	501	501
q12	860	602	596	596
q13	11289	3147	3120	3120
q14	319	275	294	275
q15	595	499	509	499
q16	719	653	643	643
q17	2006	1752	1719	1719
q18	8340	7723	7448	7448
q19	1914	1804	1727	1727
q20	2155	1896	1855	1855
q21	5798	5627	5525	5525
q22	676	578	608	578
Total cold run time: 73961 ms
Total hot run time: 61797 ms

[doris-robot]

TPC-DS: Total hot run time: 196272 ms

machine: 'aliyun_ecs.c7a.8xlarge_32C64G'
scripts: https://github.com/apache/doris/tree/master/tools/tpcds-tools
TPC-DS sf100 test result on commit bcd1c7a2499102a6679da420744c4c56f694960c, data reload: false

query1	1281	988	966	966
query2	6243	2352	2320	2320
query3	11023	4784	4630	4630
query4	33047	23712	23138	23138
query5	4331	462	474	462
query6	285	197	189	189
query7	3987	314	309	309
query8	314	228	227	227
query9	9529	2708	2714	2708
query10	473	255	248	248
query11	17813	15113	15197	15113
query12	159	108	104	104
query13	1600	438	430	430
query14	9529	7149	6931	6931
query15	256	217	198	198
query16	7922	445	464	445
query17	1576	593	626	593
query18	2112	328	347	328
query19	272	171	174	171
query20	143	115	113	113
query21	209	111	108	108
query22	4739	4612	4444	4444
query23	34576	34184	33903	33903
query24	10397	2574	2500	2500
query25	655	396	387	387
query26	1148	153	158	153
query27	3068	328	332	328
query28	7739	2471	2493	2471
query29	857	427	415	415
query30	222	146	149	146
query31	1078	869	833	833
query32	88	55	57	55
query33	770	292	285	285
query34	1002	519	519	519
query35	915	774	738	738
query36	1090	965	968	965
query37	137	77	80	77
query38	4248	4260	4452	4260
query39	1503	1460	1499	1460
query40	210	105	100	100
query41	44	47	47	47
query42	114	99	104	99
query43	541	498	500	498
query44	1468	840	847	840
query45	188	173	169	169
query46	1200	745	712	712
query47	2058	1933	1970	1933
query48	429	334	332	332
query49	1005	391	408	391
query50	850	390	391	390
query51	7158	7286	7301	7286
query52	99	93	92	92
query53	260	188	192	188
query54	1132	409	404	404
query55	89	85	85	85
query56	275	249	236	236
query57	1274	1159	1184	1159
query58	235	231	241	231
query59	3431	3105	3191	3105
query60	280	261	249	249
query61	110	109	110	109
query62	864	686	659	659
query63	218	197	182	182
query64	3986	681	665	665
query65	3237	3157	3196	3157
query66	839	305	304	304
query67	16004	15572	15524	15524
query68	5077	559	563	559
query69	479	251	250	250
query70	1214	1112	1051	1051
query71	436	256	271	256
query72	7066	4048	4028	4028
query73	783	359	356	356
query74	10298	8824	8922	8824
query75	3646	2642	2686	2642
query76	3592	1089	1125	1089
query77	585	279	303	279
query78	10144	9295	9370	9295
query79	1701	614	609	609
query80	763	426	425	425
query81	509	281	236	236
query82	1073	119	120	119
query83	180	141	142	141
query84	236	71	73	71
query85	1602	302	314	302
query86	479	282	303	282
query87	4708	4347	4514	4347
query88	3919	2213	2148	2148
query89	436	286	287	286
query90	2057	195	190	190
query91	141	117	104	104
query92	66	50	50	50
query93	2544	570	553	553
query94	869	292	280	280
query95	351	258	253	253
query96	624	283	274	274
query97	2828	2660	2691	2660
query98	226	196	195	195
query99	1585	1302	1323	1302
Total cold run time: 304151 ms
Total hot run time: 196272 ms

[doris-robot]

ClickBench: Total hot run time: 33.04 s

machine: 'aliyun_ecs.c7a.8xlarge_32C64G'
scripts: https://github.com/apache/doris/tree/master/tools/clickbench-tools
ClickBench test result on commit bcd1c7a2499102a6679da420744c4c56f694960c, data reload: false

query1	0.03	0.03	0.03
query2	0.06	0.04	0.03
query3	0.23	0.07	0.08
query4	1.60	0.11	0.11
query5	0.41	0.41	0.41
query6	1.17	0.65	0.65
query7	0.02	0.01	0.02
query8	0.04	0.03	0.03
query9	0.59	0.50	0.51
query10	0.55	0.57	0.55
query11	0.15	0.11	0.11
query12	0.13	0.11	0.11
query13	0.61	0.61	0.60
query14	2.72	2.75	2.73
query15	0.91	0.82	0.82
query16	0.38	0.39	0.38
query17	0.98	0.98	1.03
query18	0.23	0.21	0.20
query19	1.97	1.84	2.00
query20	0.02	0.01	0.01
query21	15.36	0.60	0.60
query22	2.51	2.55	2.27
query23	16.81	0.96	0.80
query24	3.58	0.97	1.68
query25	0.27	0.18	0.18
query26	0.42	0.13	0.14
query27	0.03	0.04	0.04
query28	9.97	1.14	1.07
query29	12.57	3.21	3.22
query30	0.24	0.06	0.06
query31	2.86	0.38	0.37
query32	3.27	0.46	0.45
query33	3.10	3.07	3.16
query34	16.64	4.42	4.43
query35	4.48	4.48	4.46
query36	0.69	0.47	0.47
query37	0.09	0.06	0.06
query38	0.05	0.04	0.03
query39	0.04	0.03	0.03
query40	0.17	0.12	0.12
query41	0.08	0.02	0.03
query42	0.04	0.02	0.02
query43	0.03	0.02	0.03
Total cold run time: 106.1 s
Total hot run time: 33.04 s