Don't delay DTMF in core bridge while listening for DTMF features

Don't delay DTMF in core bridge while listening for DTMF features

This patch is mostly the work of Olle Johansson. I did some cleanup and
added the silence generating code if transmit_silence is set.

When a channel listens for DTMF in the core bridge, the outbound DTMF is not
sent until we have received DTMF_END. For a long DTMF, this is a disaster. We
send 4 seconds of DTMF to Asterisk, which sends no audio for those 4 seconds.
Some products see this delay and the time skew on RTP packets that results and
start ignoring the audio that is sent afterward.

With this change, the DTMF_BEGIN frame is inspected and checked. If it matches
a feature code, we wait for DTMF_END and activate the feature as before. If
transmit_silence=yes in asterisk.conf, silence is sent if we paritally match a
multi-digit feature. If it doesn't match a feature, the frame is forwarded
along with the DTMF_END without delay. By doing it this way, DTMF is not delayed.

(closes issue #15642)
Reported by: jasonshugart
Patches:
      issue_15652_dtmf_ast-1.4.patch.txt uploaded by twilson (license 396)
Tested by: globalnetinc, jde

(closes issue #16625)
Reported by: sharvanek

Review: https://reviewboard.asterisk.org/r/1092/
Review: https://reviewboard.asterisk.org/r/1125/

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/1.4@310888 65c4cc65-6c06-0410-ace0-fbb531ad65f3