jsonrpc: allow multiple commands at once.

We now keep multiple commands for a json_connection, and an array of
json_streams.

When a command wants to write something, we allocate a new json_stream
at the end of the array.

We always output from the first available json_stream; once that
command has finished, we free that and move to the next.  Once all are
done, we wake the reader.

This means we won't read a new command if output is still pending, but
as most commands don't start writing until they're ready to write
everything, we still get command parallelism.

In particular, you can now 'waitinvoice' and 'delinvoice' and it will
work even though the 'waitinvoice' blocks.

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

lightningd/json_stream.c

@@ -23,7 +23,9 @@ struct json_stream {
 
 	/* Who is io_writing from this buffer now: NULL if nobody is. */
 	struct io_conn *reader;
-	struct io_plan *(*reader_cb)(struct io_conn *conn, void *arg);
+	struct io_plan *(*reader_cb)(struct io_conn *conn,
+				     struct json_stream *js,
+				     void *arg);
 	void *reader_arg;
 	size_t len_read;
 
@@ -263,7 +265,7 @@ static struct io_plan *json_stream_output_write(struct io_conn *conn,
 		/* We're not doing io_write now, unset. */
 		js->reader = NULL;
 		if (!json_stream_still_writing(js))
-			return js->reader_cb(conn, js->reader_arg);
+			return js->reader_cb(conn, js, js->reader_arg);
 		return io_out_wait(conn, js, json_stream_output_write, js);
 	}
 
@@ -276,6 +278,7 @@ static struct io_plan *json_stream_output_write(struct io_conn *conn,
 struct io_plan *json_stream_output_(struct json_stream *js,
 				    struct io_conn *conn,
 				    struct io_plan *(*cb)(struct io_conn *conn,
+							  struct json_stream *js,
 							  void *arg),
 				    void *arg)
 {