Make the time consuming of split table more stable

[nolouch]

Development Task

Because split and scatter tasks are asynchronous, and currently the batch size is too small (after #18191)

tidb/store/tikv/split_region.go

Lines 34 to 36 in d94920e

	const splitBatchRegionLimit = 16

which causes many split and scatter tasks at times, which have two problems:

• at the same time will cause multiple conflicts on the same region, and then report epoch not match, we need more time to retry.
  

• it is very likely to cause a split from a scattering region, resulting in many 4 replicas regions, which will lead to the time-consuming of split table be unstable. We need to ensure that all regions that need to be broken up are split at once and then scatter at once.