Reduce the memory footprint of large pending-trigger-event lists, as per my

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.