MM-10502: Only cluster master should run job schedulers. (#9174)

* MM-10502: Only cluster master should run job schedulers.

* Use sync.Map for thread safety.

* Fix tests.

app/cluster.go

@@ -0,0 +1,29 @@
+// Copyright (c) 2018-present Mattermost, Inc. All Rights Reserved.
+// See License.txt for license information.
+
+package app
+
+import "github.com/mattermost/mattermost-server/model"
+
+// Registers a given function to be called when the cluster leader may have changed. Returns a unique ID for the
+// listener which can later be used to remove it. If clustering is not enabled in this build, the callback will never
+// be called.
+func (a *App) AddClusterLeaderChangedListener(listener func()) string {
+	id := model.NewId()
+	a.clusterLeaderListeners.Store(id, listener)
+	return id
+}
+
+// Removes a listener function by the unique ID returned when AddConfigListener was called
+func (a *App) RemoveClusterLeaderChangedListener(id string) {
+	a.clusterLeaderListeners.Delete(id)
+}
+
+func (a *App) InvokeClusterLeaderChangedListeners() {
+	a.Go(func() {
+		a.clusterLeaderListeners.Range(func(_, listener interface{}) bool {
+			listener.(func())()
+			return true
+		})
+	})
+}