MXS-862: Do first part of authentication in MySQLBackend

The first message exchange between the server and the client will almost
always contain the same data. If the server is going to change
authentication methods, it will send an AuthSwitchRequest packet instead
of the OK/ERR packet that it would normally send. Only after this point
the authenticator modules actually need to do something.

In the case of the default 'mysql_native_password' plugin, the only thing
that the plugin needs to do is to check whether the server responded with
an OK packet.

server/modules/include/mysql_client_server_protocol.h

@@ -383,4 +383,7 @@ mxs_auth_state_t gw_send_backend_auth(DCB *dcb);
 /** Write an OK packet to a DCB */
 int mxs_mysql_send_ok(DCB *dcb, int sequence, int affected_rows, const char* message);
 
+/** Check for OK packet */
+bool mxs_mysql_is_ok_packet(GWBUF *buffer);
+
 #endif /** _MYSQL_PROTOCOL_H */