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/RunChromaprint.cxx

@@ -84,6 +84,8 @@ public:
 		 input_plugins_init(config, io_thread.GetEventLoop()),
 		 decoder_plugins_init(config)
 	{
+		io_thread.Start();
+
 		pcm_convert_global_init(config);
 	}
 };