presplit: send split_region request to PD instead of TiKV

[AndreMouche]

When we find a transaction try to insert into a region with more than 100000 items, it will do split the region to avoid too much write workload into a single region:

client-go/txnkv/transaction/2pc.go

Lines 866 to 877 in e30a3e6

	preSplitDetectThresholdVal := atomic.LoadUint32(&preSplitDetectThreshold)
	for _, group := range groups {
	if uint32(group.mutations.Len()) >= preSplitDetectThresholdVal {
	logutil.BgLogger().Info("2PC detect large amount of mutations on a single region",
	zap.Uint64("region", group.region.GetID()),
	zap.Int("mutations count", group.mutations.Len()),
	zap.Uint64("startTS", c.startTS))
	if c.preSplitRegion(bo.GetCtx(), group) {
	didPreSplit = true
	}
	}
	}

However, here we send split request to TiKV.

client-go/tikv/split_region.go

Lines 144 to 149 in e30a3e6

	req := tikvrpc.NewRequest(tikvrpc.CmdSplitRegion, &kvrpcpb.SplitRegionRequest{
	SplitKeys: batch.Keys,
	}, kvrpcpb.Context{
	Priority: kvrpcpb.CommandPri_Normal,
	RequestSource: util.RequestSourceFromCtx(bo.GetCtx()),
	})

I think we should send the split-request to PD since PD will only apply one operator to a region each time, which will help use to avoid the issue(tikv/tikv#12410) caused by two operators run on the same region concurrently.