channel.c: Resolve issue with receiving SIP INFO packets for DTMF

The problem is essentially the same as in ASTERISK~28245. Besides
the direct media scenario we have an additional scenario where a
special client is involved. This device mutes audio by default in
transmit direction (no rtp frames) and activates audio only by a
foot switch. In this situation dtmf input (pin for conferences,
transfer features codes , etc) using SIP INFO mode is not
understood properly especially when SIP INFO messages are sent
quickly.

This patch ensures that SIP INFO frames are properly queued and
processed in the above scenario. The patch also corrects situations
where successive dtmf events are received quicker than the
signalled event duration (plus minimum gap/pause) allows, i.e. DTMF
events have to be buffered in the ast channel read queue and
emulation has to be processed asynchronously at slower speed.

Reported by: Thomas Arimont
patches:
  trigger_dtmf_emulation.patch submitted by Thomas Arimont (license 5525)

Change-Id: I309bf61dd065c9978c8e48f5b9a936ab47de64c2