Main: use the IOThread for outputs and mixers

The main EventLoop can block for a long time while a client's command
runs, and is therefore inappropriate for internal engine I/O.  This
fixes a serious regression for at least the "httpd" output, which used
to be hard-coded for the IOThread, but now receives the main EventLoop
as an initialization parameter.

For the mixers, this doesn't make much of a difference.  They are not
latency critical.

src/Main.cxx

@@ -519,7 +519,7 @@ try {
 
 	command_init();
 
-	instance->partition->outputs.Configure(instance->event_loop,
+	instance->partition->outputs.Configure(io_thread_get(),
 					       config.replay_gain,
 					       instance->partition->pc);
 	instance->partition->UpdateEffectiveReplayGainMode();