Fix timing issue (race) with poke/pong for very close peers that can cause a...

Fix timing issue (race) with poke/pong for very close peers that can cause a peer to be declared unreachable (issue #7396 reported by stevedavies)


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@35264 65c4cc65-6c06-0410-ace0-fbb531ad65f3