Each DCB needs to be added to the owning thread's list so that they can be
iterated through. As sessions always have a client DCB, the sessions don't
need to be added to a similar per thread list.
This change fixes a problem with dcb_hangup_foreach that the removal of
the list manager introduced. Now the hangup events are properly injected
for the DCBs that connect to the server in question.
30927455ef