Allow observers.sqlite to be configured in a separate working directory

[CharlieC3]

It's my understanding the observers.sqlite holds registered predicates on who to send event data to and the scope of that data. When taking backups and restoring from a backup, we often don't want to restore an old observers.sqlite file, since the registered predicates from the backup may have changed since then, and may be old.

Ideally the observers.sqlite file could be configured to be written and read from a different working directory, so that we can take separate snapshots of two different disks, and only restore the data we're interested in.