paymod: Iterate through the routehints in order

We store an offset of the current routehint in the modifier data. It gets
incremented on retry, and it gets reset to 0 on split. This is because once we
split we have a different amount and a previously unusable routehint becomes
usable again.

plugins/libplugin-pay.h

@@ -296,6 +296,11 @@ struct routehints_data {
 	/* Current routehint, if any. */
 	struct route_info *current_routehint;
 
+	/* Position of the current routehint in the routehints
+	 * array. Inherited and incremented on child payments and reset on
+	 * split. */
+	int offset;
+
 	/* We modify the CLTV in the getroute call, so we need to remember
 	 * what the final cltv delta was so we re-apply it correctly. */
 	u32 final_cltv;