In the CutOver phase wait events up to lock timeout, the _del table lock to be not release, cause DB not writable. Introduced in #755

when gh-ost appling binlog, pauses for a period of time, and then resumes appling binlog and row copy.
At this time, the row copy is completed and start the cutOver phase.
binlog apply delay, causes timeout waiting for events up to lock thread and then lock was not released on rollback, causing the DB to be locked and cannot be written.

gh-ost execution log :
> 2021-06-20 17:03:23 INFO Row copy complete
> Copy: 594148/594148 100.0%; Applied: 1366; Backlog: 0/1000; Time: 11m38s(total), 11m38s(copy); streamer: mysql-bin.008173:15450343; Lag: 0.01s, State: migrating; ETA: due
> 2021-06-20 17:03:23 INFO Grabbing voluntary lock: gh-ost.8642188.lock
> 2021-06-20 17:03:23 INFO Setting LOCK timeout as 6 seconds
> 2021-06-20 17:03:23 INFO Looking for magic cut-over table
> 2021-06-20 17:03:23 INFO Creating magic cut-over table `test`.`_test_1_1624179105_del`
> Copy: 594148/594148 100.0%; Applied: 1366; Backlog: 0/1000; Time: 11m38s(total), 11m38s(copy); streamer: mysql-bin.008173:15508966; Lag: 0.01s, State: migrating; ETA: due
> 2021-06-20 17:03:23 INFO Magic cut-over table created
> 2021-06-20 17:03:23 INFO Locking `test`.`test_1`, `test`.`_test_1_1624179105_del`
> 2021-06-20 17:03:23 INFO Tables locked
> 2021-06-20 17:03:23 INFO Session locking original & magic tables is 8642188
> 2021-06-20 17:03:23 INFO Writing changelog state: AllEventsUpToLockProcessed:1624179803921417462
> 2021-06-20 17:03:23 INFO Waiting for events up to lock
> Copy: 594148/594148 100.0%; Applied: 1366; Backlog: 2/1000; Time: 11m38s(total), 11m38s(copy); streamer: mysql-bin.008173:15830960; Lag: 0.01s, State: migrating; ETA: due
> Copy: 594148/594148 100.0%; Applied: 1479; Backlog: 38/1000; Time: 11m41s(total), 11m38s(copy); streamer: mysql-bin.008173:44351489; Lag: 0.01s, State: migrating;
> 2021-06-20 17:03:26 ERROR Timeout while waiting for events up to lock
> 2021-06-20 17:03:26 ERROR 2021-06-20 17:03:26 ERROR Timeout while waiting for events up to lock
> 2021-06-20 17:03:26 INFO Looking for magic cut-over table
> 2021-06-20 17:03:26 INFO Will now proceed to drop magic table and unlock tables
> 2021-06-20 17:03:26 INFO Dropping magic cut-over table
> 2021-06-20 17:03:26 INFO Dropping magic cut-over table
> 2021-06-20 17:03:26 INFO Dropping table `test`.`_test_1_1624179105_del`


the above sync.Once implementation logic, the one that is grabbed first holds the m.Lock() lock, and then another thread will wait until the previous thread After the execution is complete, change the o.done status from 0 to 1, and the subsequent thread can exit without having to execute.

In gh-ost during rollback operation,if the atomicCutOver() --> defer func() --> DropAtomicCutOverSentryTableIfExists() acquires the lock first, and the drop _del table will wait for the _del table lock, and the in the AtomicCutOverMagicLock() drop _del operation will Blocked，causes the DB to be unwritable more than 300s.

Thank you!