Cache catalog lookup data across groups in ordered-set aggregates.

The initial commit of ordered-set aggregates just did all the setup work
afresh each time the aggregate function is started up.  But in a GROUP BY
query, the catalog lookups need not be repeated for each group, since the
column datatypes and sort information won't change.  When there are many
small groups, this makes for a useful, though not huge, performance
improvement.  Per suggestion from Andrew Gierth.

Profiling of these cases suggests that it might be profitable to avoid
duplicate lookups within tuplesort startup as well; but changing the
tuplesort APIs would have much broader impact, so I left that for
another day.