In particular, do not flush all debug logs still in memory

- catch OperationFailure
- reset transaction manager when leaving backup mode
- send appropriate target tid to a storage that updates a outdated cell
- clean up partition table when leaving BACKINGUP state unexpectedly
- make sure all readable cells of a partition have the same 'backup_tid'
  if they have the same data, so that we know when internal replication is
  finished when leaving backup mode
- fix storage not finished internal replication when leaving backup mode

Found by pylint.