gitea.com experienced the corrupted LevelQueue bug again.
I think the problem is clear now: if the keys in LevelDB went
out-of-sync, the LevelQueue itself doesn't have the ability to recover,
eg:
* LevelQueue.Len() reports 100
* LevelQueue.LPop() reports ErrNotFound = errors.New("no key found")
So it needs to dive into the LevelDB to remove all keys to recover the
corrupted LevelQueue.
More comments are in TestCorruptedLevelQueue.
