forestalls potential overflow when the same table (or other object, but
usually tables) is accessed by very many successive queries within a single
transaction.  Per report from Michael Milligan.

Back-patch to 8.0, which is as far back as the patch conveniently applies.
There have been no reports of overflow in pre-8.3 releases, but clearly the
risk existed all along.  (Michael's report suggests that 8.3 may consume lock
counts faster than prior releases, but with no test case to look at it's hard
to be sure about that.  Widening the counts seems a good future-proofing
measure in any event.)