mon: only learn peer addresses when monmap == 0

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>
(cherry picked from commit 7bd2104acfeff0c9aa5e648d82ed372f901f767f)
(cherry picked from commit 1996fd89fb3165a63449b135e05841579695aabd)