mds: encode quiesce payload on demand

mds: encode quiesce payload on demand

Leonid was trying to avoid re-marshalling the data multiple times or extra data
copies.  Unfortunately, the messenger interface requires that a payload can be
recomputed for message retries. See ProtocolV2::requeue_sent.

We can keep decode optimized because the payload bufferlist won't change once
the message is received.

Fixes: https://tracker.ceph.com/issues/67406
Signed-off-by: Patrick Donnelly <pdonnell@redhat.com>