astobj2: Fix rbtree duplicate handling.

OBJ_PARTIAL_KEY searching a rbtree did not find all possible matches if
the container did not accept duplicates.

Added matching node bias to indicate which matching node is being searched
for: first, last, any.


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@384616 65c4cc65-6c06-0410-ace0-fbb531ad65f3