mon: adjust mon sync, extra_probe_peers to use addrvec

mon: adjust mon sync, extra_probe_peers to use addrvec

The peer addr stuff via asok is a bit fragile because the user must
provide an exact addrvec matching the mon to avoid some weirdness, but
it's rarely used, and the fix would be some robustness/tolerance in the
messenger that is a bigger project than this.

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