Adding caching layer to some posts calls (#4779)

einterfaces/cluster.go

@@ -14,6 +14,8 @@ type ClusterInterface interface {
 	GetClusterStats() ([]*model.ClusterStats, *model.AppError)
 	RemoveAllSessionsForUserId(userId string)
 	InvalidateCacheForUser(userId string)
+	InvalidateCacheForChannel(channelId string)
+	InvalidateCacheForChannelPosts(channelId string)
 	Publish(event *model.WebSocketEvent)
 	UpdateStatus(status *model.Status)
 	GetLogs() ([]string, *model.AppError)