pgsql: Add idle_session_timeout.

Add idle_session_timeout.

This GUC variable works much like idle_in_transaction_session_timeout,
in that it kills sessions that have waited too long for a new client
query.  But it applies when we're not in a transaction, rather than
when we are.

Li Japin, reviewed by David Johnston and Hayato Kuroda, some
fixes by me

Discussion: https://postgr.es/m/763A0689-F189-459E-946F-F0EC4458980B@hotmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/9877374bef76ef03923f6aa8b955f2dbcbe6c2c7

Modified Files
--------------
doc/src/sgml/config.sgml                      | 51 +++++++++++++++++++++++----
src/backend/storage/lmgr/proc.c               |  1 +
src/backend/tcop/postgres.c                   | 48 +++++++++++++++++++++----
src/backend/utils/errcodes.txt                |  1 +
src/backend/utils/init/globals.c              |  1 +
src/backend/utils/init/postinit.c             | 10 ++++++
src/backend/utils/misc/guc.c                  | 13 ++++++-
src/backend/utils/misc/postgresql.conf.sample |  1 +
src/include/miscadmin.h                       |  1 +
src/include/storage/proc.h                    |  1 +
src/include/utils/timeout.h                   |  3 +-
11 files changed, 115 insertions(+), 16 deletions(-)