java: intro services for JNI local reference frame

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>
java: intro JavaObject::delete_local_ref

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>
Merge: FFI with Java - more local refs support

Pull-Request: #457
Reviewed-by: Jean Privat <jean@pryen.org>
Reviewed-by: Lucas Bajolet <r4pass@hotmail.com>