pgsql: Add hooks for session start and session end

Add hooks for session start and session end

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

Discussion: https://postgr.es/m/20170720204733.40f2b7eb.nagata@sraoss.co.jp

Fabrízio de Royes Mello and Yugo Nagata
Reviewed by Michael Paquier and Aleksandr Parfenov

Branch
------
master

Details
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https://git.postgresql.org/pg/commitdiff/cd8ce3a22c0b48d32ffe6543837ba3bb647ac2b2

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 | 21 ++++
src/test/modules/test_session_hooks/README | 2 +
.../expected/test_session_hooks.out | 31 +++++
.../modules/test_session_hooks/session_hooks.conf | 2 +
.../test_session_hooks/sql/test_session_hooks.sql | 12 ++
.../test_session_hooks/test_session_hooks--1.0.sql | 4 +
.../test_session_hooks/test_session_hooks.c | 134 +++++++++++++++++++++
.../test_session_hooks/test_session_hooks.control | 3 +
13 files changed, 233 insertions(+)