v2writable: support purging messages from git entirely
Purging existing messages is fairly straightforward since we can
take advantage of Xapian and lookup the git object_id with it.
Unfortunately, purging an already "removed" message (which is
no longer in Xapian) is not as easy and we'll need to expose
->purge_oids to purge by the git object_id (currently SHA-1).
Furthermore, we expire reflogs and prune in hopes a dumb HTTP
client won't get the object.