osd: discard messages on connections without sessions

When we mark_down a Connection from an old OSD, we clear the con's ref
for the Session.  However, there may be a racing delivery of a message on
that Connection.

See #9295
Signed-off-by: Sage Weil <sage@redhat.com>
osd: tolerate lack of Session in ms_fast_preprocess

See #9295.

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