Separate "restore backup" from "restore snapshot"

[mehaase]

There is one restore method in the interface that serves double duty: it restores full database backups (including data) and it also restores snapshots (DDL only). Backups can often be done more efficiently than a snapshot, i.e. a SQLite database can be backed up by copying a file and Postgres has a binary database backup format. However, because there is only one restore command, backups must be written as SQL files. If there were separate "restore backup" and "restore snapshot" methods, then "restore backup" could use the more efficient approach, and "restore snapshot" would use SQL files.