channeld: Code to implement splicing

Update the lightningd <-> channeld interface with lots of new commands to needed to facilitate spicing.

Implement the channeld splicing protocol leveraging the interactivetx protocol.

Implement lightningd’s channel_control to support channeld in its splicing efforts.

Changelog-Added: Added the features to enable splicing & resizing of active channels.

lightningd/lightningd.h

@@ -241,6 +241,9 @@ struct lightningd {
 	/* Outstanding wait commands */
 	struct list_head wait_commands;
 
+	/* Outstanding splice commands. */
+	struct list_head splice_commands;
+
 	/* Maintained by invoices.c */
 	struct invoices *invoices;