osd: streamline writing new maps to disk, somewhat
osd: streamline writing new maps to disk, somewhat

There is still an apply_transaction that should be replaced with a queue
if/when we are smarter about map caching so that a call to get_map on the
prior epoch won't need to go to disk.  Until then, the implicit flush
in apply_transaction is needed.

ajnelson/ceph,ajnelson/ceph,ajnelson/ceph,ajnelson/ceph,ajnelson/ceph,ajnelson/ceph