Add ability to pass arbitrary data to the ao2_callback_fn (called from

ao2_callback and ao2_find).  Currently, passing OBJ_POINTER to either
of these mandates that the passed 'arg' is a hashable object, making
searching for an ao2 object based on outside criteria difficult.

Reviewed by Russell and Mark M. via ReviewBoard:
    http://reviewboard.digium.com/r/36/


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@155401 65c4cc65-6c06-0410-ace0-fbb531ad65f3