Move reauthentication to authenticators

Currently the only situation where a user needs to be authenticated after
the initial authentication is when a COM_CHANGE_USER is being
executed. This was previously handled by directly calling a function in
the MySQLAuth authenticator.

The new entry in the API of the authenticators is very specific to MySQL
and should be reviewed once other protocols are added.

server/modules/include/mysql_auth.h

@@ -1,50 +0,0 @@
-#pragma once
-#ifndef _MYSQL_AUTH_H
-#define _MYSQL_AUTH_H
-/*
- * Copyright (c) 2016 MariaDB Corporation Ab
- *
- * Use of this software is governed by the Business Source License included
- * in the LICENSE.TXT file and at www.mariadb.com/bsl.
- *
- * Change Date: 2019-07-01
- *
- * On the date above, in accordance with the Business Source License, use
- * of this software will be governed by version 2 or later of the General
- * Public License.
- */
-
-/*
- * @verbatim
- * Revision History
- *
- * Date         Who                     Description
- * 02/02/2016   Martin Brampton         Initial implementation
- *
- * @endverbatim
- */
-
-#include <maxscale/cdefs.h>
-#include <maxscale/dcb.h>
-#include <maxscale/buffer.h>
-#include <stdint.h>
-#include <maxscale/protocol/mysql.h>
-
-MXS_BEGIN_DECLS
-
-int gw_check_mysql_scramble_data(DCB *dcb,
-                                 uint8_t *token,
-                                 unsigned int token_len,
-                                 uint8_t *scramble,
-                                 unsigned int scramble_len,
-                                 char *username,
-                                 uint8_t *stage1_hash);
-int check_db_name_after_auth(DCB *dcb, char *database, int auth_ret);
-int gw_find_mysql_user_password_sha1(
-    char *username,
-    uint8_t *gateway_password,
-    DCB *dcb);
-
-MXS_END_DECLS
-
-#endif /** _MYSQL_AUTH_H */