Change last_inactive_time to inactive_since in pg_replication_slots.

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