Fix race condition in invalidating obsolete replication slots

Fix race condition in invalidating obsolete replication slots

The code added to mark replication slots invalid in commit c6550776394e
had the race condition that a slot can be dropped or advanced
concurrently with checkpointer trying to invalidate it.  Rewrite the
code to close those races.

The changes to ReplicationSlotAcquire's API added with c6550776394e are
not necessary anymore.  To avoid an ABI break in released branches, this
commit leaves that unchanged; it'll be changed in a master-only commit
separately.

Backpatch to 13, where this code first appeared.

Reported-by: Andres Freund <andres@anarazel.de>
Author: Andres Freund <andres@anarazel.de>
Author: Álvaro Herrera <alvherre@alvh.no-ip.org>
Discussion: https://postgr.es/m/20210408001037.wfmk6jud36auhfqm@alap3.anarazel.de