jsonrpc: populate ld->jsonrpc ourselves, so we can use it.

Next patch will call commands to get usage inside jsonrpc_new(): to do
this it will need access to ld->jsonrpc, so we can't use the current
pattern.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

lightningd/lightningd.c

@@ -207,7 +207,7 @@ static struct lightningd *new_lightningd(const tal_t *ctx)
 	 *  lightningd needs to have something to put those in. This
 	 *  is that :-)
 	 */
-	ld->jsonrpc = jsonrpc_new(ld, ld);
+	jsonrpc_setup(ld);
 
 	/*~ We run a number of plugins (subprocesses that we talk JSON-RPC with)
 	 *alongside this process. This allows us to have an easy way for users