pgsql: Add hooks for session start and session end, take two

Add hooks for session start and session end, take two

These hooks can be used in loadable modules.  A simple test module is
included.

The first attempt was done with cd8ce3a but we lacked handling for
NO_INSTALLCHECK in the MSVC scripts (problem solved afterwards by
431f1599) so the buildfarm got angry.  This also fixes a couple of
issues noticed upon review compared to the first attempt, so the code
has slightly changed, resulting in a more simple test module.

Author: Fabrízio de Royes Mello, Yugo Nagata
Reviewed-by: Andrew Dunstan, Michael Paquier, Aleksandr Parfenov
Discussion: https://postgr.es/m/20170720204733.40f2b7eb.nagata@sraoss.co.jp
Discussion: https://postgr.es/m/20190823042602.GB5275@paquier.xyz

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/e788bd924c19e296bd34316e30e3ba1b68354e64

Modified Files
--------------
src/backend/tcop/postgres.c                        |   6 +
src/backend/utils/init/postinit.c                  |   6 +
src/include/tcop/tcopprot.h                        |   7 +
src/test/modules/Makefile                          |   1 +
src/test/modules/test_session_hooks/.gitignore     |   4 +
src/test/modules/test_session_hooks/Makefile       |  23 ++++
src/test/modules/test_session_hooks/README         |  11 ++
.../expected/test_session_hooks.out                |  37 ++++++
.../modules/test_session_hooks/session_hooks.conf  |   2 +
.../test_session_hooks/sql/test_session_hooks.sql  |  19 +++
.../test_session_hooks/test_session_hooks.c        | 146 +++++++++++++++++++++
11 files changed, 262 insertions(+)