Perform less setup work for AFTER triggers at transaction start.

Perform less setup work for AFTER triggers at transaction start.

Testing reveals that the memory allocation we do at transaction start
has small but measurable overhead on simple transactions.  To cut
down on that overhead, defer some of that work to the point when
AFTER triggers are first used, thus avoiding it altogether if they
never are.

Patch by me.  Review by Andres Freund.