mon: only learn peer addresses when monmap == 0

It is only safe to dynamically update the address for a peer mon in our
monmap if we are in the midst of the initial quorum formation (i.e.,
monmap.epoch == 0).  If it is a later epoch, we have formed our initial
quorum and any and all monmap changes need to be agreed upon by the quorum
and committed via paxos.

Fixes: #7212
Signed-off-by: Sage Weil <sage@inktank.com>
mon: tell MonmapMonitor first about winning an election

It is important in the bootstrap case that the very first paxos round
also codify the contents of the monmap itself in order to avoid any manner
of confusing scenarios where subsequent elections are called and people
try to recover and modify paxos without agreeing on who the quorum
participants are.

Signed-off-by: Sage Weil <sage@inktank.com>