Improve statistics estimation to make some use of DISTINCT in sub-queries.

Improve statistics estimation to make some use of DISTINCT in sub-queries.

Formerly, we just punted when trying to estimate stats for variables coming
out of sub-queries using DISTINCT, on the grounds that whatever stats we
might have for underlying table columns would be inapplicable.  But if the
sub-query has only one DISTINCT column, we can consider its output variable
as being unique, which is useful information all by itself.  The scope of
this improvement is pretty narrow, but it costs nearly nothing, so we might
as well do it.  Per discussion with Andres Freund.

This patch differs from the draft I submitted yesterday in updating various
comments about vardata.isunique (to reflect its extended meaning) and in
tweaking the interaction with security_barrier views.  There does not seem
to be a reason why we can't use this sort of knowledge even when the
sub-query is such a view.