Support background sync of missing refs

A recent change added support for synchronously fetching missing
refs for a change, however, if something happened to the git repo
(eg, it was garbage collected or completely removed) then gertty
won't be able to work offline (not to mention will be very slow).

Add db columns to store the fetch locations and a sync job that
will verify that all of the commits gertty needs (for open changes
at least) are on disk, and if not, will fetch them.

Change-Id: If608302b6efacceb9ac3630b7ac402cf65ae6d68