Changed back the first part of gw_MySQLWrite_backend so that it is protected by dcb->authlock which must wait until authentication phase is complete. If authentication fails, the error is handled before next query starts its execution.
MAX-10, Transaction support for MaxScale.
Naive implementation, which routes all statements to master between BEGIN|START TRANSACTION <options> and ROLLBACK|COMMIT .
Merge branch 'MAX-10' of https://github.com/skysql/MaxScale into MAX-10

Conflicts:
	server/modules/protocol/mysql_backend.c