When receiving a STUN binding request send one out as the Google Talk client uses...

When receiving a STUN binding request send one out as the Google Talk client uses this as a method to determine if the remote party is still reachable or not.

Failure to do this results in the Google Talk client ignoring RTP packets after a specific period of time. This is also done as a result of receiving a STUN binding request so that the username information can be used from the inbound request, thus not requiring it to be stored on a per candidate basis.

(closes issue ASTERISK-20107)
Reported by: Malcolm Davenport

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@369858 65c4cc65-6c06-0410-ace0-fbb531ad65f3