BUG/MINOR: stick-table: make sure never to miss a process_table_expire update

BUG/MINOR: stick-table: make sure never to miss a process_table_expire update

In stktable_requeue_exp(), there's a tiny race at the beginning during
which we check the task's expiration date to decide whether or not to
wake process_table_expire() up. During this race, the task might just
have finished running on its owner thread and we can miss a task_queue()
opportunity, which probably explains why during testing it seldom happens
that a few entries are left at the end.

Let's perform a CAS to confirm the value is still the same before
leaving. This way we're certain that our value has been seen at least
once.

This should be backported to 3.2.