paymod: Maintain a list of current and past payments

We need to keep them around so we can inspect them later. We'll also need a
background cleanup every once in a while to free some memory. More on that in
a future commit.

plugins/libplugin-pay.h

@@ -103,6 +103,7 @@ enum payment_step {
 struct payment {
 	/* The command that triggered this payment. */
 	struct command *cmd;
+	struct list_node list;
 
 	const char *json_buffer;
 	const jsmntok_t *json_toks;