Backup metadata cleanup

After a backup is complete or aborted, its config and working state remains in the database. This is useful so that `fdbbackup status` can report things about the most recently started backup, and in theory (though there is no UI for it) it could report things about backups older than that.

The config and state for a backup would be small if not for the backup snapshot file map, which is filled during a snapshot and cleared at the start of a new snapshot. This subspace under a backup's configuration is not used for anything once a backup is complete or aborted, so at the very least it should be cleared when the backup transactions to any final state.

Start by tracing the fdbbackup code that records backup configuration and snapshot file-map state, then follow the transaction paths that move a backup to a final state. Confirm how completed and aborted backups are represented, and consider the work done when the unused snapshot file map is cleared without breaking fdbbackup status reporting.

Written by the indexing model from the issue text.