Case expression pushdown

Hi.

This patch allows pushing case expressions to foreign servers, so that
more types of updates could be executed directly.

For example, without patch:

EXPLAIN (VERBOSE, COSTS OFF)
UPDATE ft2 d SET c2 = CASE WHEN c2 > 0 THEN c2 ELSE 0 END
WHERE c1 > 1000;
QUERY PLAN
-----------------------------------------------------------------------------------------------------------------------
Update on public.ft2 d
Remote SQL: UPDATE "S 1"."T 1" SET c2 = $2 WHERE ctid = $1
-> Foreign Scan on public.ft2 d
Output: CASE WHEN (c2 > 0) THEN c2 ELSE 0 END, ctid, d.*
Remote SQL: SELECT "C 1", c2, c3, c4, c5, c6, c7, c8, ctid FROM
"S 1"."T 1" WHERE (("C 1" > 1000)) FOR UPDATE

EXPLAIN (VERBOSE, COSTS OFF)
UPDATE ft2 d SET c2 = CASE WHEN c2 > 0 THEN c2 ELSE 0 END
WHERE c1 > 1000;
QUERY PLAN
----------------------------------------------------------------------------------------------------------------
Update on public.ft2 d
-> Foreign Update on public.ft2 d
Remote SQL: UPDATE "S 1"."T 1" SET c2 = (CASE WHEN (c2 > 0)
THEN c2 ELSE 0 END) WHERE (("C 1" > 1000))

--
Best regards,
Alexander Pyhalov,
Postgres Professional