Issue 487 db

This is ready for initial review. Thanks. 
oh, ID implies a temporal order (from looking at the query below)? What about a boolean field called `current`, and then enforce a unique constraint on `(current, type)`? This then allows us to add entries, and change the current "active" mount easily. The specific benefit is that, given two or more mounts for a data type, we may want to shift active writes from one mount to another in the event of degraded performance, maintenance, etc. We can then easily shift back once the issue has been resolved