Improve EXPLAIN ANALYZE to show the time spent in each trigger when

Improve EXPLAIN ANALYZE to show the time spent in each trigger when
executing a statement that fires triggers.  Formerly this time was
included in "Total runtime" but not otherwise accounted for.
As a side benefit, we avoid re-opening relations when firing non-deferred
AFTER triggers, because the trigger code can re-use the main executor's
ResultRelInfo data structure.