generate roundtrip delay requests and responses

added response to roundtrip delay requests from opposite side
added roundtrip delay request sending to opposite side after answer,
added options for sending request (interval between request and 
count of unreplied requests before forced call hangup)

(closes issue #16976)
Reported by: vmikhelson
Patches:
      rtdr-1.6.0-2.patch uploaded by may213 (license 454)
Tested by: vmikhelson, may213



git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@252277 65c4cc65-6c06-0410-ace0-fbb531ad65f3