event/TimeoutMonitor: eliminate support for periodic events

No caller needs this.  Fixes use-after-free after returning from
Client::OnTimeout().

src/event/TimeoutMonitor.cxx

@@ -45,21 +45,17 @@ TimeoutMonitor::ScheduleSeconds(unsigned s)
 	source = loop.AddTimeoutSeconds(s, Callback, this);
 }
 
-bool
+void
 TimeoutMonitor::Run()
 {
-	bool result = OnTimeout();
-	if (!result && source != nullptr) {
-		g_source_unref(source);
-		source = nullptr;
-	}
-
-	return result;
+	Cancel();
+	OnTimeout();
 }
 
 gboolean
 TimeoutMonitor::Callback(gpointer data)
 {
 	TimeoutMonitor &monitor = *(TimeoutMonitor *)data;
-	return monitor.Run();
+	monitor.Run();
+	return false;
 }

src/event/TimeoutMonitor.hxx

@@ -47,13 +47,10 @@ public:
 	void Cancel();
 
 protected:
-	/**
-	 * @return true reschedules the timeout again
-	 */
-	virtual bool OnTimeout() = 0;
+	virtual void OnTimeout() = 0;
 
 private:
-	bool Run();
+	void Run();
 	static gboolean Callback(gpointer data);
 };