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/protocol/MySQL/MySQLClient/mysql_client.c

@@ -52,7 +52,6 @@
 #include <maxscale/alloc.h>
 #include <maxscale/log_manager.h>
 #include <maxscale/protocol/mysql.h>
-#include <mysql_auth.h>
 #include <maxscale/ssl.h>
 #include <maxscale/poll.h>
 #include <maxscale/modinfo.h>