Fix leaking of small spilled subtransactions during logical decoding.

When, during logical decoding, a transaction gets too big, it's
contents get spilled to disk. Not just the top-transaction gets
spilled, but *also* all of its subtransactions, even if they're not
that large themselves.  Unfortunately we didn't clean up
such small spilled subtransactions from disk.

Fix that, by keeping better track of whether a transaction has been
spilled to disk.

Author: Andres Freund
Reported-By: Dmitriy Sarafannikov, Fabrízio de Royes Mello
Discussion:
    https://postgr.es/m/1457621358.355011041@f382.i.mail.ru
    https://postgr.es/m/CAFcNs+qNMhNYii4nxpO6gqsndiyxNDYV0S=JNq0v_sEE+9PHXg@mail.gmail.com
Backpatch: 9.4-, where logical decoding was introduced

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