WL#1966: SET SESSION CHARACTERISTICS

SET SESSION TRANSACTION ISOLATION LEVEL is not standard.  
  
Instead, allow this, which is functionally the same:  
  
SET SESSION CHARACTERISTICS AS ISOLATION LEVEL  
{ READ UNCOMMITTED | READ COMMITTED | REPEATABLE READ | SERIALIZABLE }  
  
Translate "SET SESSION TRANSACTION" to "SET SESSION CHARACTERISTICS AS".  
 
All clauses that we use for START TRANSACTION or SET TRANSACTION 
should be considered as potential clauses for SET SESSION CHARACTERISTICS, 
including the new one: WITH CONSISTENT SNAPSHOT.