Introduce a new API call ast_channel_search_locked, which iterates through the

channel list calling a caller-defined callback.  The callback returns non-zero
if a match is found.  This should speed up some of the code that I committed
earlier today in chan_sip (which is also updated by this commit).

Reviewed by russellb and kpfleming via ReviewBoard:
	http://reviewboard.digium.com/r/28/


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@154429 65c4cc65-6c06-0410-ace0-fbb531ad65f3