Backup repository database at the end of each gc.

[andrewchambers]

If/when we add optional parity information to the on disk chunks we should also move bupstash.sqlite3 to bupstash.sqlite3.prev at the end of each garbage collection using the sqlite3 backup api. We can optionally add parity information to this copy.

This means we have a usable metadata backup to protect users from corruption of the metadata.

[andrewchambers]

So we no longer have an sqlite3 database in the repository, but we should still consider copying the oplog somewhere safe at the end of gc.

[andrewchambers]

I would very much like to add this for 1.0 - It seems like an important feature.

[andrewchambers]

We now have BUPSTASH_KEEP_WAL which is worth keeping in mind for this sort of thing.