[COMMITTERS] pgsql: Shut down Gather's children before shutting down Gather itself.

Shut down Gather's children before shutting down Gather itself.

It turns out that the original shutdown order here does not work well.
Multiple people attempting to develop further parallel query patches
have discovered that they need to do cleanup before the DSM goes away,
and you can't do that if the parent node gets cleaned up first.

Patch by me, reviewed by KaiGai Kohei and Dilip Kumar.

Discussion: http://postgr.es/m/CA+TgmoY6bOc1YnhcAQnMfCBDbsJzROQ3sYxSAL-SYB5tMJcTKg@mail.gmail.com
Discussion: http://postgr.es/m/9A28C8860F777E439AA12E8AEA7694F8012AEB82@BPXM15GP.gisp.nec.co.jp
Discussion: http://postgr.es/m/CA+TgmoYuPOc=+xrG1v0fCsoLbKAab9F1ddOeaaiLMzKOiBar1Q@mail.gmail.com

Branch
------
master

Details
-------
http://git.postgresql.org/pg/commitdiff/acf555bc53acb589b5a2827e65d655fa8c9adee0

Modified Files
--------------
src/backend/executor/execProcnode.c | 4 +++-
src/backend/executor/nodeGather.c   | 2 +-
2 files changed, 4 insertions(+), 2 deletions(-)