Allow logical decoding on standbys

Unsurprisingly, this requires wal_level = logical to be set on the primary and
standby. The infrastructure added in 26669757b6a ensures that slots are
invalidated if the primary's wal_level is lowered.

Creating a slot on a standby waits for a xl_running_xact record to be
processed. If the primary is idle (and thus not emitting xl_running_xact
records), that can take a while.  To make that faster, this commit also
introduces the pg_log_standby_snapshot() function. By executing it on the
primary, completion of slot creation on the standby can be accelerated.

Note that logical decoding on a standby does not itself enforce that required
catalog rows are not removed. The user has to use physical replication slots +
hot_standby_feedback or other measures to prevent that. If catalog rows
required for a slot are removed, the slot is invalidated.

See 6af1793954e for an overall design of logical decoding on a standby.

Bumps catversion, for the addition of the pg_log_standby_snapshot() function.

Author: "Drouvot, Bertrand" <[email protected]>
Author: Andres Freund <[email protected]> (in an older version)
Author: Amit Khandekar <[email protected]> (in an older version)
Reviewed-by: Andres Freund <[email protected]>
Reviewed-by: FabrÌzio de Royes Mello <[email protected]>
Reviewed-by: Amit Kapila <[email protected]>
Reviewed-By: Robert Haas <[email protected]>

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