Avoid use of TopMemoryContext for resource owner cleanup in portals

Hi,

Whilst working on the stack-based instrumentation patch [0] which adds
a new resource owner that gets set up during a query's execution, I
encountered the following challenge:

When allocating memory related to a resource, that is intended to be
accessed during resource owner cleanup on abort, one cannot use a
memory context that's below the active portal (e.g. the executor
context), but must instead chose a longer-lived context, often
TopMemoryContext.

This is caused by the fact that At(Sub)Abort_Portals currently frees
all "subsidiary" memory of failed portals (i.e. failed portal memory
child contexts), and will be called in Abort(Sub)Transaction before
the ResourceOwnerRelease calls.

There appears to be no clear reason why the freeing of subsidiary
portal memory is being done before resource owner release - one could
argue that freeing memory earlier allows a later allocation to re-use
it, but the only relevant case I could find was
RecordTransactionAbort, and that is already handled with the
pre-allocated TransactionAbortContext to make sure we don't fail
allocations in out-of-memory aborts.

Other non-portal users of the ResOwner infrastructure don't suffer
from this problem, as they typically have a memory context set up that
survives the abort.

If we separate out the freeing of this subsidiary portal memory to run
separately, after resource owner cleanup is done (0001 patch), we can
remove a handful of uses of TopMemoryContext from the tree in LLVM
JIT, WaitEventSet and OpenSSL (0002 patch, passes CI), and make it
much less likely that new resource owner code accidentally leaks
because it uses the top memory context and missed a pfree.

It also happens to make things significantly easier for the
stack-based instrumentation patch, since we could rely on the executor
context to free memory allocations that need to be accessed during
abort (to propagate instrumentation data up the stack).

Thoughts?

Thanks,
Lukas

[0]: https://www.postgresql.org/message-id/flat/CAP53PkzdBK8VJ1fS4AZ481LgMN8f9mJiC39ZRHqkFUSYq6KWmg%40mail.gmail.com

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Lukas Fittl