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: http://svn.asterisk.org/svn/asterisk/trunk@384616 f38db490-d61c-443f-a65b-d21fe96a405b