Merge team/russell/frame_caching

There are some situations in Asterisk where ast_frame and/or iax_frame
structures are rapidly allocatted and freed (at least 50 times per second
for one call).

This code significantly improves the performance of ast_frame_header_new(), 
ast_frdup(), ast_frfree(), iax_frame_new(), and iax_frame_free() by keeping
a thread-local cache of these structures and using frames from the cache 
whenever possible instead of calling malloc/free every time.

This commit also converts the ast_frame and iax_frame structures to use the
linked list macros.


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@41278 65c4cc65-6c06-0410-ace0-fbb531ad65f3