[COMMITTERS] pgsql: Avoid passing function pointers across process boundaries.

Avoid passing function pointers across process boundaries.

This back-patches commit 32470825d36d99a81347ee36c181d609c952c061
into 9.6, primarily to make buildfarm member culicidae happy.
Unlike the HEAD patch, avoid changing the existing API of
CreateParallelContext; instead we just switch to using
CreateParallelContextForExternalFunction, even for core functions.

Petr Jelinek, with a bunch of basically-cosmetic adjustments by me

Discussion: https://postgr.es/m/548f9c1d-eafa-e3fa-9da8-f0cc2f654e60@2ndquadrant.com

Branch
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REL9_6_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/9c225acf0b97a7a3a5ca1a12ee0e89c98cf16442

Modified Files
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src/backend/access/transam/README.parallel |   2 +-
src/backend/access/transam/parallel.c      | 145 +++++++++++++++++++++--------
src/backend/executor/execParallel.c        |   7 +-
src/backend/postmaster/bgworker.c          | 103 +++++++++++---------
src/include/executor/execParallel.h        |   2 +
5 files changed, 167 insertions(+), 92 deletions(-)