Redesign API presented by nodeAgg.c for ordered-set and similar aggregates.

The previous design exposed the input and output ExprContexts of the
Agg plan node, but work on grouping sets has suggested that we'll regret
doing that.  Instead provide more narrowly-defined APIs that can be
implemented in multiple ways, namely a way to get a short-term memory
context and a way to register an aggregate shutdown callback.

Back-patch to 9.4 where the bad APIs were introduced, since we don't
want third-party code using these APIs and then having to change in 9.5.

Andrew Gierth