mds: notify all mds about prepared snaptable update

mds: notify all mds about prepared snaptable update

After snaptable update get prepared, push the update preparation to
all active snaptable clients, then send reply to update initiator.
By this way, the initiator know that all mds have record the update
preparation in their cache. When committing the snaptable update,
the initiator notifies all mds about the commit. Bystander mds'
snaptable cache get synchronized when it receives the notification.

Signed-off-by: "Yan, Zheng" <zyan@redhat.com>