pgsql: Allow compute_query_id to be set to 'auto' and make it default

Allow compute_query_id to be set to 'auto' and make it default

Allowing only on/off meant that all either all existing configuration
guides would become obsolete if we disabled it by default, or that we
would have to accept a performance loss in the default config if we
enabled it by default.  By allowing 'auto' as a middle ground, the
performance cost is only paid by those who enable pg_stat_statements and
similar modules.

I only edited the release notes to comment-out a paragraph that is now
factually wrong; further edits are probably needed to describe the
related change in more detail.

Author: Julien Rouhaud <rjuju123@gmail.com>
Reviewed-by: Justin Pryzby <pryzby@telsasoft.com>
Discussion: https://postgr.es/m/20210513002623.eugftm4nk2lvvks3@nol

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/cafde58b337e007cb6a719f5ab4dd6459d932a39

Modified Files
--------------
contrib/pg_stat_statements/pg_stat_statements.c    |  6 ++++
contrib/pg_stat_statements/pg_stat_statements.conf |  1 -
doc/src/sgml/config.sgml                           |  9 +++--
doc/src/sgml/pgstatstatements.sgml                 | 14 +++-----
doc/src/sgml/release-14.sgml                       |  2 ++
src/backend/commands/explain.c                     |  2 +-
src/backend/parser/analyze.c                       |  4 +--
src/backend/postmaster/postmaster.c                |  3 ++
src/backend/tcop/postgres.c                        |  2 +-
src/backend/utils/misc/guc.c                       | 38 ++++++++++++++++------
src/backend/utils/misc/postgresql.conf.sample      |  2 +-
src/backend/utils/misc/queryjumble.c               | 21 ++++++++++++
src/include/utils/guc.h                            |  1 -
src/include/utils/queryjumble.h                    | 33 +++++++++++++++++--
14 files changed, 108 insertions(+), 30 deletions(-)