Fix multiple problems in postgres_fdw query cancellation logic.

Fix multiple problems in postgres_fdw query cancellation logic.

First, even if we cancel a query, we still have to roll back the
containing transaction; otherwise, the session will be left in a
failed transaction state.

Second, we need to support canceling queries whe aborting a
subtransaction as well as when aborting a toplevel transaction.

Etsuro Fujita, reviewed by Michael Paquier