msgr: generate reset event on mark_down too

Previously I was avoiding this because the caller could know what they are
doing and break the ref cycle themselves, but in reality most don't, and I
don't think it is worth the complexity to try to make them when generating
a reset event is easy and neatly solves the problem of leaked Session
and associated Connections on the OSDs.

Signed-off-by: Sage Weil <sage@inktank.com>
osd: make ms_handle_reset debug more useful

Signed-off-by: Sage Weil <sage@inktank.com>
common/RefCountedObject: take cct for debugging
osd: debug Session refs
debug refs on Connection
debug Pipe refs
debug message refs (incoming ones at least)