Send a SRCCHANGE indication when we masquerade

Send a SRCCHANGE indication when we masquerade

Masquerading a channel means that the src of the audio is potentially
changing, so send a SRCCHANGE so that RTP-based media streams can get
a new SSRC generated to reflect the change. Original patch by addix
(along with lots of testing--thanks!).

(closes issue #17007)
Reported by: addix
Patches:
      1001-reset-SSRC-original-channel.diff uploaded by addix (license 1006)
      srcchange.diff uploaded by twilson (license 396)
Tested by: addix, twilson

Review: https://reviewboard.asterisk.org/r/862/

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/1.4@282430 65c4cc65-6c06-0410-ace0-fbb531ad65f3