pgsql: Add plan_cache_mode setting

Add plan_cache_mode setting

This allows overriding the choice of custom or generic plan.

Author: Pavel Stehule <pavel.stehule@gmail.com>
Discussion:
https://www.postgresql.org/message-id/flat/CAFj8pRAGLaiEm8ur5DWEBo7qHRWTk9HxkuUAz00CZZtJj-LkCA%40mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/f7cb2842bf47715133b40e4a503f35dbe60d1b72

Modified Files
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doc/src/sgml/config.sgml                      | 30 +++++++++++
src/backend/utils/cache/plancache.c           |  8 +++
src/backend/utils/misc/guc.c                  | 19 +++++++
src/backend/utils/misc/postgresql.conf.sample |  1 +
src/include/utils/plancache.h                 | 11 ++++
src/test/regress/expected/plancache.out       | 77 +++++++++++++++++++++++++++
src/test/regress/sql/plancache.sql            | 33 ++++++++++++
7 files changed, 179 insertions(+)