Main: start IO threads before initializing the rest

This fixes a regression triggered by commit e309941646e0ff1b6 -
IORING_SETUP_SINGLE_ISSUER broke because io_uring_setup() was called
fromm the main thread, yet io_uring_submit() in the I/O thread.  This
is because the I/O thread was not yet started when
InitUringInputPlugin() was called, and BlockingCall() was invoked
synchronously in the main thread.  This has always been wrong, but was
never noticed.

src/Main.cxx

@@ -329,6 +329,9 @@ MainConfigured(const CommandLineOptions &options,
 	Instance instance;
 	global_instance = &instance;
 
+	instance.io_thread.Start();
+	instance.rtio_thread.Start();
+
 #ifdef ENABLE_NEIGHBOR_PLUGINS
 	instance.neighbors = std::make_unique<NeighborGlue>();
 	instance.neighbors->Init(raw_config,
@@ -421,9 +424,6 @@ MainConfigured(const CommandLineOptions &options,
 	};
 #endif
 
-	instance.io_thread.Start();
-	instance.rtio_thread.Start();
-
 #ifdef ENABLE_NEIGHBOR_PLUGINS
 	if (instance.neighbors != nullptr)
 		instance.neighbors->Open();