event/TimerEvent: add ScheduleEarlier()

src/event/TimerEvent.cxx

@@ -27,3 +27,10 @@ TimerEvent::Schedule(Event::Duration d) noexcept
 
 	loop.AddTimer(*this, d);
 }
+
+void
+TimerEvent::ScheduleEarlier(Event::Duration d) noexcept
+{
+	if (!IsPending() || loop.SteadyNow() + d < due)
+		Schedule(d);
+}

src/event/TimerEvent.hxx

@@ -53,8 +53,7 @@ class TimerEvent final
 
 public:
 	TimerEvent(EventLoop &_loop, Callback _callback) noexcept
-		:loop(_loop), callback(_callback) {
-	}
+		:loop(_loop), callback(_callback) {}
 
 	auto &GetEventLoop() const noexcept {
 		return loop;
@@ -66,6 +65,12 @@ public:
 
 	void Schedule(Event::Duration d) noexcept;
 
+	/**
+	 * Like Schedule(), but is a no-op if there is a due time
+	 * earlier than the given one.
+	 */
+	void ScheduleEarlier(Event::Duration d) noexcept;
+
 	void Cancel() noexcept {
 		unlink();
 	}