Allow ON UPDATE/DELETE SET DEFAULT plans to be cached.

Once upon a time, somebody was worried that cached RI plans wouldn't get
remade with new default values after ALTER TABLE ... SET DEFAULT, so they
didn't allow caching of plans for ON UPDATE/DELETE SET DEFAULT actions.
That time is long gone, though (and even at the time I doubt this was the
greatest hazard posed by ALTER TABLE...).  So allow these triggers to cache
their plans just like the others.

The cache_plan argument to ri_PlanCheck is now vestigial, since there
are no callers that don't pass "true"; but I left it alone in case there
is any future need for it.