Use `mDDLTableVersion` key to control backfill related tables.

[Defined2014]

Enhancement

split InitDDLJobTables to two functions. one for create ddl tables, another for create backfill related tables. It could avoid get database list during bootstrap.

tidb/session/session.go

Lines 3107 to 3130 in 337af61

	func InitDDLJobTables(store kv.Storage) error {
	return kv.RunInNewTxn(kv.WithInternalSourceType(context.Background(), kv.InternalTxnDDL), store, true, func(ctx context.Context, txn kv.Transaction) error {
	t := meta.NewMeta(txn)
	exists, err := t.CheckDDLTableExists()
	if err != nil {
	return errors.Trace(err)
	}
	dbID, err := t.CreateMySQLDatabaseIfNotExists()
	if err != nil {
	return err
	}
	if exists {
	return initBackfillJobTables(store, t, dbID)
	}
	if err = createAndSplitTables(store, t, dbID, DDLJobTables); err != nil {
	return err
	}
	if err = initBackfillJobTables(store, t, dbID); err != nil {
	return err
	}
	return t.SetDDLTables()
	})
	}