Add semi-join pushdown to postgres_fdw

Hi.

It's possible to extend deparsing in postgres_fdw, so that we can push 
down semi-joins, which doesn't refer to inner reltarget. This allows
us to push down joins in queries like

SELECT * FROM ft1 t1 WHERE t1.c1 < 10 AND t1.c3 IN (SELECT c3 FROM ft2 
t2 WHERE date(c5) = '1970-01-17'::date);


EXPLAIN (VERBOSE, COSTS OFF) SELECT * FROM ft1 t1 WHERE t1.c1 < 10 AND 
t1.c3 IN (SELECT c3 FROM ft2 t2 WHERE date(c5) = '1970-01-17'::date);
                                                                          
                                         QUERY PLAN

-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Foreign Scan
    Output: t1.c1, t1.c2, t1.c3, t1.c4, t1.c5, t1.c6, t1.c7, t1.c8
    Relations: (public.ft1 t1) SEMI JOIN (public.ft2 t2)
    Remote SQL: SELECT r1."C 1", r1.c2, r1.c3, r1.c4, r1.c5, r1.c6, 
r1.c7, r1.c8 FROM "S 1"."T 1" r1 WHERE ((r1."C 1" < 10)) AND (EXISTS 
(SELECT NULL FROM "S 1"."T 1" r3 WHERE ((date(r3.c5) = 
'1970-01-17'::date)) AND ((r1.c3 = r3.c3))))

Deparsing semi-joins leads to generating (text) conditions like 'EXISTS 
(SELECT NULL FROM  inner_rel WHERE join_conds) . Such conditions are 
generated in deparseFromExprForRel() and distributed to nearest WHERE, 
where they are added to the list of and clauses.

-- 
Best regards,
Alexander Pyhalov,
Postgres Professional