Softmix: Fix crash when switching from softmix to another bridge technology.

The crash is caused by a race condition when switching between native RTP
and softmix bridging technologies.  In this situation, the bridging
technology is switched from native RTP to softmix, and then back to native
RTP fast enough that the softmix private data gets destroyed before the
softmix mixing thread gets started.

Thanks to Kinsey Moore for the crash analysis.

* Fix race condition when starting the softmix mixing thread and switching
to another bridge technology.

(closes issue ASTERISK-22678)
Reported by: John Bigelow
Patches:
      jira_asterisk_22678_v12.patch (license #5621) patch uploaded by rmudgett
Tested by: John Bigelow
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Merged revisions 400849 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: http://svn.asterisk.org/svn/asterisk/trunk@400850 f38db490-d61c-443f-a65b-d21fe96a405b
Fixed merge tracking for r400360, which was somehow lost

git-svn-id: http://svn.asterisk.org/svn/asterisk/trunk@400851 f38db490-d61c-443f-a65b-d21fe96a405b
Multiple revisions 400508,400842-400843,400848

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  r400508 | dlee | 2013-10-03 23:54:51 -0500 (Thu, 03 Oct 2013) | 1 line
  
  Corrected response class for stopPlayback
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  r400842 | dlee | 2013-10-10 14:23:24 -0500 (Thu, 10 Oct 2013) | 1 line
  
  Correct some ARI wiki rendering errors
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  r400843 | dlee | 2013-10-10 14:26:19 -0500 (Thu, 10 Oct 2013) | 1 line
  
  Updated /play resource docs. The playback of http: resources isn't implemented... yet
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  r400848 | dlee | 2013-10-11 11:18:46 -0500 (Fri, 11 Oct 2013) | 5 lines
  
  Fix a stupid copy/paste error in ARI docs.
  
  Patches:
      ari-doc-patch.txt uploaded by jbigelow (license 5091)
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Merged revisions 400508,400842-400843,400848 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: http://svn.asterisk.org/svn/asterisk/trunk@400852 f38db490-d61c-443f-a65b-d21fe96a405b
channel.h: whitespace changes.
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Merged revisions 400854 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: http://svn.asterisk.org/svn/asterisk/trunk@400855 f38db490-d61c-443f-a65b-d21fe96a405b