plugin: Move list of notification topics to each plugin

We want to ensure that plugins register their topics before sending
any notification, so we need to remember which plugin registered which
topics.

lightningd/plugin.h

@@ -93,6 +93,10 @@ struct plugin {
 	/* Parameters for dynamically-started plugins. */
 	const char *parambuf;
 	const jsmntok_t *params;
+
+	/* Notification topics that this plugin has registered with us
+	 * and that other plugins may subscribe to. */
+	const char **notification_topics;
 };
 
 /**
@@ -128,10 +132,6 @@ struct plugins {
 	/* Whether builtin plugins should be overridden as unimportant.  */
 	bool dev_builtin_plugins_unimportant;
 #endif /* DEVELOPER */
-
-	/* Notification topics that plugins have registered with us
-	 * and that other plugins may subscribe to. */
-	const char **notification_topics;
 };
 
 /* The value of a plugin option, which can have different types.