pgsql: Add functions to wait for backend termination

Add functions to wait for backend termination

This adds a function, pg_wait_for_backend_termination(), and a new
timeout argument to pg_terminate_backend(), which will wait for the
backend to actually terminate (with or without signaling it to do so
depending on which function is called). The default behaviour of
pg_terminate_backend() remains being timeout=0 which does not waiting.
For pg_wait_for_backend_termination() the default wait is 5 seconds.

Author: Bharath Rupireddy
Reviewed-By: Fujii Masao, David Johnston, Muhammad Usama,
             Hou Zhijie, Magnus Hagander
Discussion: https://postgr.es/m/CALj2ACUBpunmyhYZw-kXCYs5NM+h6oG_7Df_Tn4mLmmUQifkqA@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/aaf043257205ec523f1ba09a3856464d17cf2281

Modified Files
--------------
doc/src/sgml/func.sgml                  |  30 +++++++-
doc/src/sgml/monitoring.sgml            |   4 ++
src/backend/catalog/system_views.sql    |  10 +++
src/backend/storage/ipc/signalfuncs.c   | 124 ++++++++++++++++++++++++++++++--
src/backend/utils/activity/wait_event.c |   3 +
src/include/catalog/pg_proc.dat         |   9 ++-
src/include/utils/wait_event.h          |   1 +
7 files changed, 174 insertions(+), 7 deletions(-)