pgsql: Change last_inactive_time to inactive_since in pg_replication_sl

Change last_inactive_time to inactive_since in pg_replication_slots.

Commit a11f330b55 added last_inactive_time to show the last time the slot
was inactive. But, it tells the last time that a currently-inactive slot
previously *WAS* active. This could be unclear, so we changed the name to
inactive_since.

Reported-by: Robert Haas
Author: Bharath Rupireddy
Reviewed-by: Bertrand Drouvot, Shveta Malik, Amit Kapila
Discussion: https://postgr.es/m/CA+Tgmob_Ta-t2ty8QrKHBGnNLrf4ZYcwhGHGFsuUoFrAEDw4sA@mail.gmail.com
Discussion: https://postgr.es/m/CALj2ACUXS0SfbHzsX8bqo+7CZhocsV52Kiu7OWGb5HVPAmJqnA@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/6d49c8d4b4f4a20eb5b4c501d78cf894fa13c0ea

Modified Files
--------------
doc/src/sgml/system-views.sgml            |  4 +-
src/backend/catalog/system_views.sql      |  2 +-
src/backend/replication/slot.c            | 17 +++++----
src/backend/replication/slotfuncs.c       |  4 +-
src/include/catalog/catversion.h          |  2 +-
src/include/catalog/pg_proc.dat           |  2 +-
src/include/replication/slot.h            |  4 +-
src/test/recovery/t/019_replslot_limit.pl | 62 +++++++++++++++----------------
src/test/regress/expected/rules.out       |  4 +-
9 files changed, 52 insertions(+), 49 deletions(-)