timer-queue: add a new timer queue helper

This allows you to schedule timeouts. It keeps a sorted list of
timeouts and uses just 1 timerfd to schedule the head of the timeout
list.

doc/tree.dox

@@ -43,6 +43,7 @@ This determines the ordering of items in Doxygen sidebar.
 \addtogroup pw_protocol
 \addtogroup pw_resource
 \addtogroup pw_thread_loop
+\addtogroup pw_timer_queue
 \addtogroup pw_work_queue
 \}