[MEDIUM] session-counters: correctly unbind the counters tracked by the backend

[MEDIUM] session-counters: correctly unbind the counters tracked by the backend

In case of HTTP keepalive processing, we want to release the counters tracked
by the backend. Till now only the second set of counters was released, while
it could have been assigned by the frontend, or the backend could also have
assigned the first set. Now we reuse to unused bits of the session flags to
mark which stick counters were assigned by the backend and to release them as
appropriate.