pgsql: Allow custom and foreign scans to have shutdown callbacks.

Allow custom and foreign scans to have shutdown callbacks.

This is expected to be useful mostly when performing such scans in
parallel, because in that case it allows (in combination with commit
acf555bc53acb589b5a2827e65d655fa8c9adee0) nodes below a Gather to get
control just before the DSM segment goes away.

KaiGai Kohei, except that I rewrote the documentation. Reviewed by
Claudio Freire.

Discussion: http://postgr.es/m/CADyhKSXJK0jUJ8rWv4AmKDhsUh124_rEn39eqgfC5D8fu6xVuw@mail.gmail.com

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/a315b967cc1bd43ecf3c10ea48b44a4fb0ff8d45

Modified Files
--------------
doc/src/sgml/custom-scan.sgml | 13 +++++++++++++
doc/src/sgml/fdwhandler.sgml | 14 ++++++++++++++
src/backend/executor/execProcnode.c | 6 ++++++
src/backend/executor/nodeCustom.c | 9 +++++++++
src/backend/executor/nodeForeignscan.c | 16 ++++++++++++++++
src/include/executor/nodeCustom.h | 1 +
src/include/executor/nodeForeignscan.h | 1 +
src/include/foreign/fdwapi.h | 2 ++
src/include/nodes/extensible.h | 1 +
9 files changed, 63 insertions(+)