OSD: for old osds, dispatch peering messages immediately

Normally, we batch up peering messages until the end of
process_peering_events to allow us to combine many notifies, etc
to the same osd into the same message.  However, old osds assume
that the actiavtion message (log or info) will be _dispatched
before the first sub_op_modify of the interval.  Thus, for those
peers, we need to send the peering messages before we drop the
pg lock, lest we issue a client repop from another thread before
activation message is sent.

Signed-off-by: Samuel Just <sam.just@inktank.com>
Reviewed-by: Sage Weil <sage@inktank.com>
(cherry picked from commit 4ae4dce5c5bb547c1ff54d07c8b70d287490cae9)