Changes to the metadata log format.

A small collection of changes to make the snapshot format a little more
rational (I hope).  The intent is to also more or less merge the formats of
the metadata log and statcache.  There will likely be other changes in the
future in the course of working on metadata scalability.  The format is not
yet finalized!

WARNING: These changes will require new tools to read the generated backup
snapshots.  The version number in the format has been updated.  This
changeset does not include the necessary changes to the snapshot-parsing
code.