Channel lock janitor -- add locks around retrieval of channel variables

(closes issue #12840)
 Reported by: pputman
 Patches: 
       app_dial_threadsafe3.patch uploaded by pputman (license 81)


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@123648 65c4cc65-6c06-0410-ace0-fbb531ad65f3