txnkv: introduce ballast object for batch-get worker

[zyguan]

Batch-get may spawn multiple goroutines to fetch keys. There is notable cost on stack growing.

After some analysis, it shows the batch-get worker requires 8KB stack size currently, that is, the stack will grow to 4KB, then 8KB for each worker groutine. To reduce the overhead, we can introduce a ballast object to let the stack grow to 8KB directly at the begining of the goroutine.

The benchmark results show that it helps improve performance.

workload	threads	baseline	this PR	diff
batchget	15	19450.06	19548.51	0.51%
batchget	30	24507.40	24914.81	1.66%
batchget	60	26797.00	27451.43	2.44%

BTW, I also tried to use the internal goroutine pool of kvstore, but the test result is not good, the overhead of selectgo is significant, and default 128 pool size is not enough to prevent new goroutines.

[ti-chi-bot]

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