rbd: use watch/notify for changes in rbd header

Send notifications when we change the rbd header (e.g. create a snapshot)
and wait for such notifications.  This allows synchronizing the snapshot
creation between different rbd clients/rools.

Signed-off-by: Yehuda Sadeh <yehuda@hq.newdream.net>
Signed-off-by: Sage Weil <sage@newdream.net>