Performance tweak to jsonIterator.

Performance tweak to jsonIterator.

Instead of storing a malloc'd copy of the key of a JSON_HASH entry,
just store a const pointer to the key string stored in the
internal osrfHash.  That way we don't have to do a malloc and
free every time we bump the iterator.

This change also requires the addition of a couple of const qualifiers
in the client code.

git-svn-id: svn://svn.open-ils.org/OpenSRF/trunk@1732 9efc2488-bf62-4759-914b-345cdb29e865