Implement autosaving of data held in memory

Add controller.autosave_interval config that specifies the frequency
which controller config, hosts, instances, users, mod packs and plugin
specific data which is held in memory should be automatically flushed to
disk if they have changed since the last time they were saved.  This
prevents data loss in the event of a server crash.

Plugins can use the onSaveData hook to save its own data at the same
time as the controller does and this obsoletes making an autosave
interval in the plugin.

Resolves #523.

packages/controller/src/BaseControllerPlugin.ts

@@ -40,6 +40,19 @@ export default class BaseControllerPlugin {
 	 */
 	async init() { }
 
+	/**
+	 * Called when the controller saves data in memory to disk
+	 *
+	 * Invoked on the configured controller.autosave_interval by the
+	 * controller and intended to be used to flush any in memory data that
+	 * has changed to disk.
+	 *
+	 * This will also be called during graceful shutdown after {@link
+	 * BaseControllerPlugin.onShutdown} have been invoked and all links have
+	 * been disconnected.
+	 */
+	async onSaveData() { }
+
 	/**
 	 * Called when the status of an instance changes
 	 *