Periodic database & attachments garbage collection: Bug #714134

See the ticket (comment #2) for more information on the thinking and
strategy here, but in a nutshell this will remove from the Geary
database all emails no longer accessible via any folder and not seen
on the server in over 30 days.  It also deletes those messages
attachment(s) and removes any empty directories in the attachment/
directory to prevent clutter.  If enough messages are garbage
collected, Geary will vacuum the database at startup, which will
lower its disk footprint and reduce fragmentation, potentially
increasing performance.