Delete TiFlash placement rules in gc worker may take a long time

[Lloyd-Pottiger]

Enhancement

Now in gc worker, we delete placement rules of TiFlash one by one. It may takes a long time when drop database or drop table with a lot of partitions. We can easily optimize it by batch delete.

tidb/pkg/store/gcworker/gc_worker.go

Lines 1547 to 1557 in 8f56847

	for _, id := range physicalTableIDs {
	// Delete pd rule
	failpoint.Inject("gcDeletePlacementRuleCounter", func() {})
	logutil.BgLogger().Info("try delete TiFlash pd rule",
	zap.Int64("tableID", id), zap.String("endKey", string(dr.EndKey)), zap.Uint64("safePoint", safePoint))
	ruleID := infosync.MakeRuleID(id)
	if err := infosync.DeleteTiFlashPlacementRule(context.Background(), "tiflash", ruleID); err != nil {
	logutil.BgLogger().Error("delete TiFlash pd rule failed when gc",
	zap.Error(err), zap.String("ruleID", ruleID), zap.Uint64("safePoint", safePoint))
	}
	}

[JaySon-Huang]

Consider backport this to release-6.5/7.1/7.5/8.1