postgres_fdw: evaluate placeholdervars on remote server

Hi,

Here is a patch for $subject. This is the same as what I proposed in
combination with a feature for full joins [1]; this would allow us to
push down left/right/full joins with PHVs to the remote and improve how
to deparse whole-row references. Since this is implemented on top of
the feature for full-joins (ie, the deparser logic for subqueries), I
proposed this on that thread, but this is slightly independent from that
feature (and we haven't discussed this in detail on that thread), so I
think it's better to start new thread. Attached is a new version, which
is created on top of [2]. I'll add this to the upcoming CF.

Best regards,
Etsuro Fujita

[1]
https://www.postgresql.org/message-id/c449261a-b033-dc02-9254-2fe5b7044795%40lab.ntt.co.jp
[2]
https://www.postgresql.org/message-id/920e660b-6fec-6022-759d-e96e37dd5984%40lab.ntt.co.jp