Avoid leaking memory during large-scale REASSIGN OWNED BY operations.

The various ALTER OWNER routines tend to leak memory in
CurrentMemoryContext.  That's not a problem when they're only called
once per command; but in this usage where we might be touching many
objects, it can amount to a serious memory leak.  Fix that by running
each call in a short-lived context.

(DROP OWNED BY likely has a similar issue, except that you'll probably
run out of lock table space before noticing.  REASSIGN is worth fixing
since for most non-table object types, it won't take any lock.)

Back-patch to all supported branches.  Unfortunately, in the back
branches this helps to only a limited extent, since the sinval message
queue bloats quite a lot in this usage before commit 3aafc030a,
consuming memory more or less comparable to what's actually leaked.
Still, it's clearly a leak with a simple fix, so we might as well fix it.

Justin Pryzby, per report from Guillaume Lelarge

Discussion: https://postgr.es/m/CAECtzeW2DAoioEGBRjR=CzHP6TdL=yosGku8qZxfX9hhtrBB0Q@mail.gmail.com

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