Add start() method to Periodic (and construct stopped)

client/src/connection.vala

@@ -112,6 +112,7 @@ namespace StudySystemClient {
 		public Worker(SessionManager session_manager) {
 			base(TASK_PERIOD_MS);
 			this.session_manager = session_manager;
+			start();
 		}
 
 		protected override void task() {

client/src/periodic.vala

@@ -6,17 +6,25 @@ namespace StudySystemClient {
 
 		protected Periodic(uint period_ms) {
 			this.period_ms = period_ms;
-			exit = false;
-			thread = new Thread<void>("Periodic task", body);
+			exit = true;
 		}
 
 		~Periodic() {
-			exit = true;
-			thread.join();
+			if (!exit) {
+				exit = true;
+				thread.join();
+			}
 		}
 
 		protected abstract void task();
 
+		public void start() {
+			if (exit) {
+				exit = false;
+				thread = new Thread<void>("Periodic task", body);
+			}
+		}
+
 		private void body() {
 			while (!exit) {
 				task();