MEDIUM: stick-table: requeue the expiration task out of the exclusive lock

MEDIUM: stick-table: requeue the expiration task out of the exclusive lock

With 48 threads, a heavily loaded table with plenty of trackers and
rules and a short expiration timer of 10ms saturates the CPU at 232k
rps. By carefully using atomic ops we can make sure that t->exp_next
and t->task->expire converge to the earliest next expiration date and
that all of this can be performed under atomic ops without any lock.
That's what this patch is doing in stktable_touch_with_exp(). This is
sufficient to double the performance and reach 470k rps.

It's worth noting that __stktable_store() uses a mix of eb32_insert()
and task_queue, and that the second part of it could possibly benefit
from this, even though sometimes it's called under a lock that was
already held.