pgsql: Track last_inactive_time in pg_replication_slots.

Track last_inactive_time in pg_replication_slots.

This commit adds a new property called last_inactive_time for slots. It is
set to 0 whenever a slot is made active/acquired and set to the current
timestamp whenever the slot is inactive/released or restored from the disk.
Note that we don't set the last_inactive_time for the slots currently being
synced from the primary to the standby because such slots are typically
inactive as decoding is not allowed on those.

The 'last_inactive_time' will be useful on production servers to debug and
analyze inactive replication slots. It will also help to know the lifetime
of a replication slot - one can know how long a streaming standby, logical
subscriber, or replication slot consumer is down.

The 'last_inactive_time' will also be useful to implement inactive
timeout-based replication slot invalidation in a future commit.

Author: Bharath Rupireddy
Reviewed-by: Bertrand Drouvot, Amit Kapila, Shveta Malik
Discussion: https://www.postgresql.org/message-id/CALj2ACW4aUe-_uFQOjdWCEN-xXoLGhmvRFnL8SNw_TZ5nJe+aw@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/a11f330b5584f2430371d68871e00f5c63735299

Modified Files
--------------
doc/src/sgml/system-views.sgml            |  10 ++
src/backend/catalog/system_views.sql      |   1 +
src/backend/replication/slot.c            |  35 +++++++
src/backend/replication/slotfuncs.c       |   7 +-
src/include/catalog/catversion.h          |   2 +-
src/include/catalog/pg_proc.dat           |   6 +-
src/include/replication/slot.h            |   3 +
src/test/recovery/t/019_replslot_limit.pl | 152 ++++++++++++++++++++++++++++++
src/test/regress/expected/rules.out       |   3 +-
9 files changed, 213 insertions(+), 6 deletions(-)