pgsql: Allow insert and update tuple routing and COPY for foreign table

Allow insert and update tuple routing and COPY for foreign tables.

Also enable this for postgres_fdw.

Etsuro Fujita, based on an earlier patch by Amit Langote. The larger
patch series of which this is a part has been reviewed by Amit
Langote, David Fetter, Maksim Milyutin, Álvaro Herrera, Stephen Frost,
and me. Minor documentation changes to the final version by me.

Discussion: http://postgr.es/m/29906a26-da12-8c86-4fb9-d8f88442f2b9@lab.ntt.co.jp

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/3d956d9562aa4811b5eaaaf5314d361c61be2ae0

Modified Files
--------------
contrib/file_fdw/input/file_fdw.source | 5 +
contrib/file_fdw/output/file_fdw.source | 9 +-
contrib/postgres_fdw/expected/postgres_fdw.out | 334 +++++++++++++++++++++++++
contrib/postgres_fdw/postgres_fdw.c | 96 +++++++
contrib/postgres_fdw/sql/postgres_fdw.sql | 237 ++++++++++++++++++
doc/src/sgml/ddl.sgml | 8 +-
doc/src/sgml/fdwhandler.sgml | 66 +++++
doc/src/sgml/ref/copy.sgml | 5 +-
doc/src/sgml/ref/update.sgml | 3 +
src/backend/commands/copy.c | 96 +++++--
src/backend/executor/execMain.c | 8 +-
src/backend/executor/execPartition.c | 103 +++++---
src/backend/executor/nodeModifyTable.c | 23 +-
src/include/executor/execPartition.h | 8 +-
src/include/foreign/fdwapi.h | 8 +
src/include/nodes/execnodes.h | 6 +
16 files changed, 924 insertions(+), 91 deletions(-)