WL#2248: Unify START TRANSACTION and SET TRANSACTION

Starting from 4.1.8 we support
START TRANSACTION [WITH CONSISTENT SNAPSHOT];
and
SET TRANSACTION ISOLATION LEVEL etc;
So if we look at SQL2003 there are two things to do:
- implement WITH CONSISTENT SNAPSHOT as a clause of SET TRANSACTION
- implement ISOLATION LEVEL as a clause of START TRANSACTION.

SET TRANSACTION clauses; START TRANSACTION; is equivalent to
START TRANSACTION clauses;
About the SET: as SQL2003 says:
"Set the characteristics of the next SQL-transaction for the SQL-agent. NOTE 411
  This statement has no effect on any SQL-transactions subsequent to the next
SQL-transaction."