osd: discriminate based on connection messenger, not peer type

Replace ->get_source().is_osd() checks and instead see if it is the
cluster_messenger so that we do not confuse ourselves when we get
legit requests from other OSDs on our public interface.

NOTE: backporting this because a mixed cluster may send OSD requests
via the client interface, even though dumpling doesn't do this.

Signed-off-by: Sage Weil <sage@inktank.com>
(cherry picked from commit a6b04c5d8ba043727a2e39a62e9d4126485bcfeb)

Conflicts:

	src/osd/OSD.cc