Improve support for composite types in PL/Python.

Allow PL/Python functions to return arrays of composite types.
Also, fix the restriction that plpy.prepare/plpy.execute couldn't
handle query parameters or result columns of composite types.

In passing, adopt a saner arrangement for where to release the
tupledesc reference counts acquired via lookup_rowtype_tupdesc.
The callers of PLyObject_ToCompositeDatum were doing the lookups,
but then the releases happened somewhere down inside subroutines
of PLyObject_ToCompositeDatum, which is bizarre and bug-prone.
Instead release in the same function that acquires the refcount.

Ed Behn and Ronan Dunklau, reviewed by Abhijit Menon-Sen
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
Don't cache per-group context across the whole query in orderedsetaggs.c.

Although nodeAgg.c currently uses the same per-group memory context for
all groups of a query, that might change in future.  Avoid assuming it.
This costs us an extra AggCheckCallContext() call per group, but that's
pretty cheap and is probably good from a safety standpoint anyway.

Back-patch to 9.4 in case any third-party code copies this logic.

Andrew Gierth
Split out the description of page-level lock as new subsection in document.

Michael Banck
Refactor pg_receivexlog main loop code, for readability.

Previously the source codes for receiving the data and for
polling the socket were included in pg_receivexlog main loop.
This commit splits out them as separate functions. This is
useful for improving the readability of main loop code and
making the future pg_receivexlog-related patch simpler.
Fix double-free bug of WAL streaming buffer in pg_receivexlog.

This bug was introduced while refactoring in commit 74cbe96.