Create DB backups without interrupting operation

[cespare]

Right now we have to stop the DB to make a backup or copy of the DB. But it would be quite easy to implement this in the DB itself: we just need to take a snapshot of the current DB static table state, and then copy all the segment files to the backup location. We need to wait on the backup to finish before deleting segments in the same way we wait for readers.

We could even do this same thing but make hard links rather than copying the files to do an almost instantaneous full DB snapshot.