Too many goroutines in commit phase affect performance

[youjiali1995]

@BusyJay found too many goroutines in commit phase affect performance a lot. The committer-concurrency is 1024. The region size on left side is smaller than right side.

It's not an apple to apple comparison, because larger region results in larger requests and batch, but the throughput of TiKV is the same.

Disregarding TiKV, too many goroutines affects GC, schedule of TSO goroutine and so on. We can improve it.

[BusyJay]

One coroutine to one transaction is a more reasonable mapping to me.

[tiancaiamao]

committer-concurrency is configurable and its default value is not 1024

[BusyJay]

committer-concurrency is configurable and its default value is not 1024

I don't think this configuration matters. It's possible to increase the transaction count while decrease this configuration to achieve similar regression.