cohort_lru: crash fix and reduce lock contention

cohort_lru: crash fix and reduce lock contention

Fixes crash induced by taking the address of the last element
of an empty intrusive list (!).

Also, introduces active queue, reducing potential for lock
contention in evict_block():

* entries are tracked on lane::active_queue when lru_refcnt > 1
** on some lane::q otherwise

Object transition between queues when lru_refcnt changes value--
a value of 0 triggers deletion, as before.

Fixes: https://tracker.ceph.com/issues/73992
Signed-off-by: Matt Benjamin <mbenjamin@redhat.com>