jsonrpc: helper to move an existing io_write.

It's a very ugly one-liner; really ccan/io should have an io_replan
for this, but it would have to be written carefully as it makes
assumptions currently about plans not changing.  In this case, we know
it's in io_write, and we're just moving a pointer.

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

lightningd/jsonrpc.c

@@ -6,6 +6,7 @@
 #include <bitcoin/script.h>
 #include <ccan/array_size/array_size.h>
 #include <ccan/err/err.h>
+#include <ccan/io/backend.h>
 #include <ccan/io/io.h>
 #include <ccan/str/hex/hex.h>
 #include <ccan/tal/str/str.h>
@@ -53,6 +54,12 @@ static void destroy_jcon(struct json_connection *jcon)
 	tal_free(jcon->log);
 }
 
+/* FIXME: This, or something prettier (io_replan?) belong in ccan/io! */
+static UNNEEDED void adjust_io_write(struct io_conn *conn, ptrdiff_t delta)
+{
+	conn->plan[IO_OUT].arg.u1.cp += delta;
+}
+
 static void json_help(struct command *cmd,
 		      const char *buffer, const jsmntok_t *params);