mon: collect mon metadata as part of election

mon: collect mon metadata as part of election

Previously each peon would send a message to teh leader with its metadata
immediately after the election was won.  However, at that point paxos
usually wasn't writeable, which meant the old update_mon_metadata() method
didn't persist reliably, updates would race, and generally speaking
metadata wasn't reliably updated.

Fix this by including metadata as part of the election ack, and persisting
the whole quorum when the election is won.  This ensures it is up to date.

Fixes: http://tracker.ceph.com/issues/20434
Signed-off-by: Sage Weil <sage@redhat.com>