MDEV-10314 : wsrep_client_thread was not set in threadpool.

Fixed threadpool_add_connection to use thd_prepare_connection()
to match thread-per-conection flow.

sql/threadpool_common.cc

@@ -148,9 +148,8 @@ int threadpool_add_connection(THD *thd)
 
   if (!setup_connection_thread_globals(thd))
   {
-    if (!login_connection(thd))
+    if (!thd_prepare_connection(thd))
     {
-      prepare_new_connection_state(thd);
       
       /* 
         Check if THD is ok, as prepare_new_connection_state()