msgr: queue reset exactly once on any connection

Use the atomic pipe link removal as a signal that we are the one failing
the con and use that to queue the reset event.

This fixes the case where we have an open, the session gets set up via the
handle_accept callback, and then race with another connection and go into
wait + close, or just close.  In that case, fault() needs to queue a reset
event to match the accept.

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