Honor inherited ownership when finishing prepared transactions

Recently started to use 2pc support in SPQR [0] for transactions that
need to modify data on several postgres(shards). So, for partially
committed 2pc we have a separate watchdog which finishes all half-done
transactions.

And we want a separate role for this watchdog. But this just fails to
finish transactions that were prepared. The reason is a two phase
check that compares user oid with tx owner oid with ==. This was the
original security restriction from [1] (valid at time), but didn't
change after [2]. Before [2], server had logic that only user himself
can do something with his object, for example dropping the table was
ONLY for owner. After [2], it use role membership etc. So, can we use
same in 2pc (prepared) tx? Is [1] sec restriction still true? I have
simple patch for this, didn't posted yet, want to hear back

[1]Discussion: https://www.postgresql.org/message-id/15495.1097685302@sss.pgh.pa.us
[2]Discussion: https://www.postgresql.org/message-id/26627.1119985659@sss.pgh.pa.us

[0] https://github.com/pg-sharding/spqr

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Best regards,
Kirill Reshke