Add a WHEN clause to CREATE TRIGGER, allowing a boolean expression to be
checked to determine whether the trigger should be fired.
 
For BEFORE triggers this is mostly a matter of spec compliance; but for AFTER
triggers it can provide a noticeable performance improvement, since queuing of
a deferred trigger event and re-fetching of the row(s) at end of statement can
be short-circuited if the trigger does not need to be fired.
 
Takahiro Itagaki, reviewed by KaiGai Kohei.