Arrange so that startup connections are handled by main Worker

When the Worker mechanism has been initialized the current_worker_id
of the calling thread is set to 0. That way, connections can be created
after Worker::init() has been called, but before the workers have been
started. Such connections will be handled by the worker that is running
in the main thread.

server/core/worker.cc

@@ -214,6 +214,16 @@ bool Worker::init()
         MXS_ERROR("Could not allocate an epoll instance.");
     }
 
+    if (this_unit.initialized)
+    {
+        // When the initialization has successfully been performed, we set the
+        // current_worker_id of this thread to 0. That way any connections that
+        // are made during service startup (after this function returns, but
+        // bofore the workes have been started) will be handled by the worker
+        // that will be running in the main thread.
+        this_thread.current_worker_id = 0;
+    }
+
     return this_unit.initialized;
 }