Add stopping instance status

Notify when an instance is in the process of stopping with a separate
status for it.  This makes handling shutdown gracefully a bit simpler in
plugin code.

packages/lib/link/messages.js

@@ -368,7 +368,8 @@ messages.listInstances = new Request({
 					"id": { type: "integer" },
 					"assigned_slave": { type: ["null", "integer"] },
 					"status": { enum: [
-						"unknown", "unassigned", "stopped", "starting", "running", "creating_save", "exporting_data",
+						"unknown", "unassigned", "stopped", "starting", "running", "stopping",
+						"creating_save", "exporting_data",
 					]},
 				},
 			},
@@ -747,7 +748,7 @@ messages.updateInstances = new Request({
 				properties: {
 					"serialized_config": { type: "object" },
 					"status": { enum: [
-						"stopped", "starting", "running", "creating_save", "exporting_data",
+						"stopped", "starting", "running", "stopping", "creating_save", "exporting_data",
 					]},
 				},
 			},
@@ -979,7 +980,7 @@ messages.instanceStatusChanged = new Event({
 	eventProperties: {
 		"instance_id": { type: "integer" },
 		"status": { enum: [
-			"stopped", "starting", "running", "creating_save", "exporting_data",
+			"stopped", "starting", "running", "stopping", "creating_save", "exporting_data",
 		]},
 	},
 });

packages/lib/plugin.js

@@ -260,6 +260,7 @@ class BaseMasterPlugin {
 	 *   slave.
 	 * - `starting`: Instance is in the process of starting up.
 	 * - `running`: Instance startup completed and is now running.
+	 * - `stopping`: Instance is in the processing stopping.
 	 * - `creating_save`: Instance is in the process of creating a save.
 	 * - `exporting_data`: Instance is in the process of exporting item
 	 *   icons and locale data.

packages/slave/slave.js

@@ -224,7 +224,7 @@ class Instance extends libLink.Link {
 	/**
 	 * Current state of the instance
 	 *
-	 * One of stopped, starting, running, creating_save and exporting_data
+	 * One of stopped, starting, running, stopping, creating_save and exporting_data
 	 *
 	 * @returns {string} instance status.
 	 */
@@ -623,6 +623,7 @@ class Instance extends libLink.Link {
 	 */
 	async stop() {
 		this._running = false;
+		this.notifyStatus("stopping");
 
 		// XXX this needs more thought to it
 		if (this.server._state === "running") {