And make caller of json_stream_forward_change_id use it, since
we're going to reuse that.
Also call json_out_finished here, so next object doesn't have a ","
prepended.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
This pass to json_stream helpers for commands outputs, but keeps
compatibility with existing plugins which use jout as of now, by not
starting/closing the "result"/"error" objects.
