Wip fs cmds

Actually, I think it's safe — although perhaps by mistake. We trigger a proposal on the OSDMap before we do on the MDS, which guarantees that the OSDMap is committed before the MDSMap change is.
That's in tick(), so it doesn't wait for the MDSMap to commit before returning, but I think we can do the same thing here and just do a wait_for_commit() or whatever the call is.