Fix: flush the rotation thread's job queue on exit

The rotation thread's job queue can transitively hold references
to a number of ltt_session objects which will prevent the session
daemon to exit as it waits for all sessions to have completed
their destruction.

This fix ensures that the job queue is flushed when activity
is observed on the quit pipe.

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