Improve vtgate logging for buffering

[deepthi]

Description

In #17294 we moved some of the buffering related logs to level 2. However, we do want to always see Starting/Stopping/Draining messages which are emitted once per shard. What we wanted to suppress were the "disruption in shard resolved" type messages which we were logging up to n^2 times if you are concurrently running PRS on n shards.

Changes in this PR

• Continue to log Starting/Stopping/Draining once per shard
• Move "keyspace event resolved" logs per-shard to level 2
• add a log line for "keyspace is consistent" per keyspace, whenever it becomes consistent

No back ports are required since the original PR was not back ported.

Related Issue(s)

#17294

Checklist

• "Backport to:" labels have been added if this change should be back-ported to release branches
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• Tests were added or are not required
• Did the new or modified tests pass consistently locally and on CI?
• Documentation was added or is not required

Deployment Notes

[vitess-bot]

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[codecov]

Codecov Report

Attention: Patch coverage is 60.00000% with 2 lines in your changes missing coverage. Please review.

Project coverage is 67.75%. Comparing base (9c6c380) to head (74a413c).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
go/vt/discovery/keyspace_events.go	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #17654      +/-   ##
==========================================
- Coverage   67.76%   67.75%   -0.02%     
==========================================
  Files        1586     1586              
  Lines      255763   255764       +1     
==========================================
- Hits       173315   173285      -30     
- Misses      82448    82479      +31     

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