pgsql: postgres_fdw: Consider foreign joining and foreign sorting toget

postgres_fdw: Consider foreign joining and foreign sorting together.

Commit ccd8f97922944566d26c7d90eb67ab7848ee9905 gave us the ability to
request that the remote side sort the data, and, later, commit
e4106b2528727c4b48639c0e12bf2f70a766b910 gave us the ability to
request that the remote side perform the join for us rather than doing
it locally. But we could not do both things at the same time: a
remote SQL query that had an ORDER BY clause would never be a join.
This commit adds that capability.

Ashutosh Bapat, reviewed by me.

Branch
------
master

Details
-------
http://git.postgresql.org/pg/commitdiff/aa09cd242fa7e3a694a31f8aed521e80d1e626a4

Modified Files
--------------
contrib/postgres_fdw/expected/postgres_fdw.out | 303 +++++++++++++------------
contrib/postgres_fdw/postgres_fdw.c | 120 +++++++---
contrib/postgres_fdw/sql/postgres_fdw.sql | 5 +
3 files changed, 240 insertions(+), 188 deletions(-)