This implements message revision tracking.  This is definitely a
work-in-progress, but I wanted to post it since I don't know when I'll
be able to work on it next (and maybe someone else can run with it in
the mean time).  I think this makes all of the necessary library-side
changes, but doesn't do anything in the CLI to expose current revision
information other than adding support for a "lastmod" query.

This series applies on top of the ghost message series, but only
because of trivial conflicts in the lists of features in database.cc
and database-private.h.  There's no code dependency.