Implement preservable jobs and configurable jobs table name

Preserved jobs, if enabled, are marked with a completed state in the database. Turning preservation off if it was previously on does not clear old jobs from the table; it just prevents new ones from being preserved.

Configuring the jobs table name requires passing the same name (and optionally, space) to both the queue driver and the migration. This is necessary because there's no way for the migration to see the driver's configuration.