mon: do not forward other mon's requests to other mons

The request forwarding infrastructure is there for client requests.
However, we (ab)use it for mon's sending MLog messages: LogClient sends an
MLog message to itself, and that is either handled locally (if leader) or
forwarded to the leader.

If that races with an election, we were forwarding an MLog from another mon
to the leader.  This is not necessary; the original MLog sender will resend
the request on election_finish() to the latest leader.

The fix is to adjust forward_request_leader() to only forward messages from
a mon if that mon is itself.

This was reproduced while testing the fix for #4748.

Signed-off-by: Sage Weil <sage@inktank.com>
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>
Merge branch 'wip-mon-fwd' into next

Reviewed-by: Greg Farnum <greg@inktank.com>