pgsql: Reduce the memory footprint of large pending-trigger-event lists,

Log Message:
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Reduce the memory footprint of large pending-trigger-event lists, as per my
recent proposal. In typical cases, we now need 12 bytes per insert or delete
event and 16 bytes per update event; previously we needed 40 bytes per
event on 32-bit hardware and 80 bytes per event on 64-bit hardware. Even
in the worst case usage pattern with a large number of distinct triggers being
fired in one query, usage is at most 32 bytes per event. It seems to be a
bit faster than the old code as well, due to reduction of palloc overhead.

This commit doesn't address the TODO item of allowing the event list to spill
to disk; rather it's trying to stave off the need for that. However, it
probably makes that task a bit easier by reducing the data structure's
dependency on pointers. It would now be practical to dump an event list to
disk by "chunks" instead of individual events.

Modified Files:
--------------
pgsql/src/backend/commands:
trigger.c (r1.237 -> r1.238)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/trigger.c?r1=1.237&r2=1.238)
pgsql/src/include/commands:
trigger.h (r1.68 -> r1.69)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/commands/trigger.h?r1=1.68&r2=1.69)