Fix a CEL LINKEDID_END race and local channel linkedids

Fix a CEL LINKEDID_END race and local channel linkedids

This patch has the ;2 channel inherit the linkedid of the ;1 channel and fixes
the race condition by no longer scanning the channel list for "other" channels
with the same linkedid. Instead, cel.c has an ao2 container of linkedid strings
and uses the refcount of the string as a counter of how many channels with the
linkedid exist. Not only does this eliminate the race condition, but it also
allows us to look up the linkedid by the hashed key instead of traversing the
entire channel list.

Review: https://reviewboard.asterisk.org/r/1895/

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/1.8@365006 65c4cc65-6c06-0410-ace0-fbb531ad65f3