Notify master about instance status update

Send updates to the master server when the running status of instances
change and add onInstanceStatusChanged hook to master plugins.

lib/plugin.js

@@ -163,6 +163,21 @@ class BaseMasterPlugin {
 	 */
 	async init() { }
 
+	/**
+	 * Called when the status of an instance changes
+	 *
+	 * Invoked when the master server has received notice from a slave that
+	 * the running status of an instance has changed.  The possible statuses
+	 * are `stopped` meaning it is not loaded or running, `initialized`
+	 * meaning the instance has initialized and is starting up the Factorio
+	 * server, and `running` which means the Factorio server it manages is
+	 * up and running.
+	 *
+	 * @param {Object} instance - the instance that changed.
+	 * @param {string} prev - the previous status of the instance.
+	 */
+	async onInstanceStatusChanged(instance, prev) { }
+
 	/**
 	 * Called before collecting Prometheus metrics
 	 *