Revert "event/Thread: start the thread in the constructor"

This reverts commit b49cfe96f4.  It was
a bad idea because it broke signal handlers.  I need to find a better
way to fix io_uring intialization.

test/DumpDatabase.cxx

@@ -28,6 +28,12 @@ class GlobalInit {
 	EventThread io_thread;
 
 public:
+	GlobalInit() {
+		io_thread.Start();
+	}
+
+	~GlobalInit() = default;
+
 	EventLoop &GetEventLoop() {
 		return io_thread.GetEventLoop();
 	}