event-loop: Track timer event sources in userspace

libwayland now uses only one file descriptor to keep track of all
the timer event sources associated with an event loop. An array-based
binary heap is used to determine which event source has the earliest
deadline.

(Previously, each timer event source had its own timerfd, making it easy
for the a process using many timer event sources to run out of file
descriptors.)

Signed-off-by: Manuel Stoeckl <code@mstoeckl.com>