Fix: ensure the ht clean pipe is empty before processing quit pipe

The ht-cleanup thread does not ensure that all data pending on its
ht_clean_pipe has been read (and processed) before processing the
"quit" event on the quit_pipe.

This causes a number of urcu hash tables to be leaked on exit of
the sessiond even though they have been pushed on the cleanup
pipe.

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