pgsql: Allow parallel custom and foreign scans.

Allow parallel custom and foreign scans.

This patch doesn't put the new infrastructure to use anywhere, and
indeed it's not clear how it could ever be used for something like
postgres_fdw which has to send an SQL query and wait for a reply,
but there might be FDWs or custom scan providers that are CPU-bound,
so let's give them a way to join club parallel.

KaiGai Kohei, reviewed by me.

Branch
------
master

Details
-------
http://git.postgresql.org/pg/commitdiff/69d34408e5e7adcef8ef2f4e9c4f2919637e9a06

Modified Files
--------------
doc/src/sgml/custom-scan.sgml          | 37 ++++++++++++++++++++
doc/src/sgml/fdwhandler.sgml           | 47 ++++++++++++++++++++++++++
src/backend/executor/execParallel.c    | 26 ++++++++++++++
src/backend/executor/nodeCustom.c      | 45 ++++++++++++++++++++++++
src/backend/executor/nodeForeignscan.c | 62 ++++++++++++++++++++++++++++++++++
src/include/executor/nodeCustom.h      | 11 ++++++
src/include/executor/nodeForeignscan.h |  8 +++++
src/include/foreign/fdwapi.h           | 14 ++++++++
src/include/nodes/execnodes.h          | 14 +++++++-
9 files changed, 263 insertions(+), 1 deletion(-)