MEDIUM: stick-table: do not take a lock to update t->current anymore.

MEDIUM: stick-table: do not take a lock to update t->current anymore.

We don't need to be protected by the table's lock when touching t->current
if we do it using atomics, and that's great because it allows us to have
a cleaner stksess_new() that doesn't require a lock either, and to avoid
manipulating pools under a lock.

That's another 1% performance gain from 2.07 to 2.10M req/s under 48
threads.