Fix some timer state corruption.

Fix some timer state corruption.

In res_timer_timerfd, handle the case that set_rate gets called while a timer
is still in continuous mode.  In this case, we want to remember the configured
rate, but not actually set it until continuous mode has been disabled.

Thanks to dvossel for finding and helping to debug the problem.

(closes issue #15080)
Reported by: dvossel
Tested by: dvossel

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@193718 65c4cc65-6c06-0410-ace0-fbb531ad65f3