Resolve issues with choppy sound when using res_timing_pthread.

The situation that caused this problem was when continuous mode was being
turned on and off while a rate was set for a timing interface.  A very easy
way to replicate this bug was to do a Playback() from behind a Local channel.
In this scenario, a rate gets set on the channel for doing file playback.
At the same time, continuous mode gets turned on and off about every 20 ms
as frames get queued on to the PBX side channel from the other side of the
Local channel.

Essentially, this module treated continuous mode and a set rate as mutually
exclusive states for the timer to be in.  When I dug deep enough, I observed
the following pattern:

   1) Set timer to tick every 20 ms.
   2) Wait almost 20 ms ...
   3) Continuous mode gets turned on for a queued up frame
   4) Continuous mode gets turned off
   5) The timer goes back to its tick per 20 ms. state but starts counting
      at 0 ms.
   6) Goto step 2.

Sometimes, res_timing_pthread would make it 20 ms and produce a timer tick,
but not most of the time.  This is what produced the choppy sound (or sometimes
no sound at all).

Now, the module treats continuous mode and a set rate as completely independent
timer modes.  They can be enabled and disabled independently of each other and
things work as expected.


(closes issue #14412)
Reported by: dome
Patches:
      issue14412.diff.txt uploaded by russell (license 2)
      issue14412-1.6.1.0.diff.txt uploaded by russell (license 2)
Tested by: DennisD, russell


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@198146 65c4cc65-6c06-0410-ace0-fbb531ad65f3