xctl: prepare for backup
xctl: prepare for backup

On backup we copy files corresponding to the most recent checkpoint.
Since xctl does not create snapshots of its log files, but instead
appends records written after checkpoint to the most recent log file,
the signature of the xctl file corresponding to the last checkpoint
equals the signature of the previous checkpoint. So upon successful
recovery from a backup we need to rotate the log to keep checkpoint and
xctl signatures in sync.

rtsisyk/tarantool,rtsisyk/tarantool,rtsisyk/tarantool,rtsisyk/tarantool