mon: fix double-forwarding check

The PaxosServiceMessage fields are no longer filled in.  Use Session::proxy_con
instead.

Signed-off-by: Sage Weil <sage@inktank.com>
mon: mark PaxosServiceMessage forward fields deprecated

These are no longer used; we manage forward state explicitly via the
Monitor sessions instead.  Mark them deprecated so we don't accidentally
rely on them.  Also, fix the annoying "mon.-1" garbage debug output that
is confusing.

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