event/Loop: non-recursive Break() implementation

Simply set the "quit" flag and wake up the thread.  This works even if
we're inside this thread.  Setting "quit" to a new value without mutex
protection is usually not safe, but good enough here.

src/event/Loop.cxx

@@ -52,10 +52,8 @@ EventLoop::~EventLoop()
 void
 EventLoop::Break()
 {
-	if (IsInside())
-		quit = true;
-	else
-		AddCall([this]() { Break(); });
+	quit = true;
+	wake_fd.Write();
 }
 
 bool