Base mainloop on pa_rtclock_now()

Base mainloop on pa_rtclock_now()

Move the mainloop to monotonic based time events.

Introduces 4 helper functions:
pa_{context,core}_rttime_{new,restart}(), that fill correctly a
timeval with the rtclock flag set if the mainloop supports it.

Both mainloop-test and mainloop-test-glib works with rt and timeval
based time events. PulseAudio and clients should be fully functional.

This patch has received several iterations, and this one as been
largely untested.

Signed-off-by: Marc-André Lureau <marca-andre.lureau@nokia.com>