json: Better handle JSON objects with duplicate names.

json: Better handle JSON objects with duplicate names.

RFC 4627 (which defines JSON) says:

    The names within an object SHOULD be unique.

In my view, this means that the treatment of duplicate names within a
JSON object is more or less up to the implementation.  Until now, the OVS
JSON parser has dealt with duplicates fairly badly: they all get shoved
into the hash table and you get one or the other value semi-randomly
(typically the one added later).  This commit makes the behavior
predictable: old values are deleted and replaced by newer values.