Simplify housekeeper shutdown

The two-part shutdown procedure for the housekeeper was not needed and
caused problems if SIGINT wasn't raised. Since the main thread returns to
the main function, a single shutdown function is all that the housekeeper
needs to function.

Moved all the shutdown related code into Housekeeper::stop to remove the
waiting for the thread in the destructor.

server/core/gateway.cc

@@ -2336,7 +2336,6 @@ int maxscale_shutdown()
     {
         service_shutdown();
         Worker::shutdown_all();
-        hkshutdown();
         log_flush_shutdown();
     }