Project IDs and per-migration backups

[sastraxi]

Allow users to keep a memory of the latest "copy" of each particular repo / migration combination.

Every so often, we'll launch a background task at the start of pgsh to look at all the databases with the same migration history, md5sum the file representing the latest migration, and see if the current database has a newer write than a different version of the same database (not really fleshed out)

For each config.project_id, we'll keep the last n

• project ID could be, by default, the name of the repo / package.json

Commands:

• backup pause
• backup on
• backup off
• backup 50G
• backup 30
• backup peek -- switch to the backup database
• backup restore [target] -- clone the backup database into a new named database (or the current one if none given)