Create iterative method for querying SRV results, and use that for finding AGI servers.

(closes issue #14775)
 Reported by: _brent_
 Patches: 
       20091215__issue14775.diff.txt uploaded by tilghman (license 14)
       hagi-5.patch uploaded by brent (license 388)
 Tested by: _brent_
 Reviewboard: https://reviewboard.asterisk.org/r/378/


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@241188 65c4cc65-6c06-0410-ace0-fbb531ad65f3