dm cache policy mq: track entries hit this 'tick' via sentinel objects

A sentinel object is placed on each level of the multiqueues.  When an
object is hit it is requeued behind the sentinel.  When the tick is
incremented we iterate through all objects behind the sentinel and
update the hit_count, then reposition the sentinel at the very back.

This saves memory by avoiding tracking the tick explicitly for every
struct entry object in the multiqueues.

Signed-off-by: Joe Thornber <ejt@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>