Fix: leak of UST app hash tables

The ht_cleanup thread is shut down before the queue of rcu
callbacks is emptied by the rcu_barrier(). Since callbacks added
by call_rcu can push hash tables through the ht_cleanup pipe, we run
into cases where the clean-up thread has been shutdown and
hash tables pushed through the clean-up pipe are leaked.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>