MXS-2355 If client is using the wrong authenticator, attempt a switch

Some SQL clients may default to a different authentication plugin than
"mysql_native_password". Since this is the only one supported by MySQL-
authenticator, the client is instructed to swap its plugin.

server/modules/protocol/MySQL/mariadbclient/mysql_client.cc

@@ -712,6 +712,7 @@ gw_read_do_authentication(DCB *dcb, GWBUF *read_buffer, int nbytes_read)
     }
 
     next_sequence++;
+    ((MYSQL_session*)(dcb->data))->next_sequence = next_sequence;
 
     /**
      * The first step in the authentication process is to extract the