Fixed database deadlock when syncer crash

[wubiaoi]

Currently, in database transaction processing, if an exception occurs without abort, there is no lock release.
When a node dies and there is no sync task, there is a divisor 0 exception in RebalanceLoad.

ERROR checker failed, host info: 10.119.48.44:8411, err: [db] Error 1205 (HY000): Lock wait timeout exceeded; try restarting transaction

panic: runtime error: integer divide by zero

goroutine 187 [running]:
github.com/selectdb/ccr_syncer/pkg/storage.RebalanceLoad(0x0, 0xb36dc8?, {0xc0006b82a0?, 0x1?, 0x30?})
        /root/src/ccr-syncer-data/pkg/storage/utils.go:59 +0xea
github.com/selectdb/ccr_syncer/pkg/storage.(*MysqlDB).RebalanceLoadFromDeadSyncers(0xc000534070, {0xc0000fb340, 0x1, 0x1})
        /root/src/ccr-syncer-data/pkg/storage/mysql.go:378 +0x1ab
github.com/selectdb/ccr_syncer/pkg/ccr.(*Checker).handleRebalance(0xc0001d4000)
        /root/src/ccr-syncer-data/pkg/ccr/checker.go:128 +0x92

[w41ter]

BTW, since a defer function is executed, please remove the other rollback invoking, eg the below lines

			if err := txn.Rollback(); err != nil {
				return xerror.Wrap(err, xerror.DB, "mysql: rebalance rollback failed.")
			}

[w41ter]

The err is a shadow variable here.

A better way is to make err as a named return value, eg:

func (s *MysqlDB) GetStampAndJobs(hostInfo string) (int64, []string, err error) {

and change the usage of err from , err := to , err = .

[wubiaoi]

done