Move the mwi send thread functionality back into the do_monitor thread so that...

Move the mwi send thread functionality back into the do_monitor thread so that it is easier to manage CID spill resources when do_monitor needs to be killed. 
(closes issue #13213)
Reported by: bbryant



git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@140246 65c4cc65-6c06-0410-ace0-fbb531ad65f3