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
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