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

Revert hooks for session start and end, take two

The location of the session end hook has been chosen so as it is
possible to allow modules to do their own transactions, however any
trying to any any subsystem which went through before_shmem_exit()
would cause issues, limiting the pluggability of the hook.

Per discussion with Tom Lane and Andres Freund.

Discussion: https://postgr.es/m/18722.1569906636@sss.pgh.pa.us

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/9555cc8d2b02c4191d67ba39f589b39b01735518

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 | 155 ---------------------
11 files changed, 271 deletions(-)