os/DBObjectMap: new version v2; drop support for upgrading from v1

- Don't support upgrade from v1 anymore
- Add upgrade for v1 -> v2

Signed-off-by: Sage Weil <sage@redhat.com>
os/FileStore: move to v4, trigger DBOjectMap upgrade

From FileStore's perspective, nothing changed from v3 to v4.  However, we
need to pass the do_upgrade flag to DBOjectMap open to trigger it's
upgrade process.

Signed-off-by: Sage Weil <sage@redhat.com>