mon: break con <-> session ref cycle in mon even if shutting down

If we get a reset during shutdown, we should still break the cycle to avoid
tripping the valgrind leak detection.  Note that we are touching no
internal Monitor state here and the locking has not changed.

Signed-off-by: Sage Weil <sage@inktank.com>
mon: mark_down session by con, not addr

We have the ConnectionRef here; use it.  This avoids generating a spurious
RESET event for the connection.

Signed-off-by: Sage Weil <sage@inktank.com>
client: mark_down by con

We have the con handy; use it.  This avoids generate a spurious RESET
event, which we do not need or do anything useful with.  Note that in this
case we are not attaching anything to the Connection priv field.

Signed-off-by: Sage Weil <sage@inktank.com>