OPTIM/MINOR: init/threads: only call protocol_enable_all() on first thread

There's no point in calling this on each and every thread since the first
thread passing there will enable the listeners, and the next ones will
simply scan all of them in turn to discover that they are already
initialized. Let's only initilize them on the first thread. This could
slightly speed up start up on very large configurations, eventhough most
of the time is still spent in the main thread binding the sockets.

A few measurements have constantly shown that this decreases the startup
time by ~0.1s for 150k listeners. Starting all of them in parallel doesn't
provide better results and can still expose some undesired races.

src/haproxy.c

@@ -2611,7 +2611,8 @@ static void *run_thread_poll_loop(void *data)
 	thread_sync_release();
 	thread_isolate();
 
-	protocol_enable_all();
+	if (tid == 0)
+		protocol_enable_all();
 
 	/* done initializing this thread, don't start before others are done */
 	thread_sync_release();