Fix bug where we truncated CLOG that was still needed by LISTEN/NOTIFY

Fix bug where we truncated CLOG that was still needed by LISTEN/NOTIFY

The async notification queue contains the XID of the sender, and when
processing notifications we call TransactionIdDidCommit() on the
XID. But we had no safeguards to prevent the CLOG segments containing
those XIDs from being truncated away. As a result, if a backend didn't
for some reason process its notifications for a long time, or when a
new backend issued LISTEN, you could get an error like:

test=# listen c21;
ERROR:  58P01: could not access status of transaction 14279685
DETAIL:  Could not open file "pg_xact/000D": No such file or directory.
LOCATION:  SlruReportIOError, slru.c:1087

To fix, make VACUUM "freeze" the XIDs in the async notification queue
before truncating the CLOG. Old XIDs are replaced with
FrozenTransactionId or InvalidTransactionId.

Note: This commit is not a full fix. A race condition remains, where a
backend is executing asyncQueueReadAllNotifications() and has just
made a local copy of an async SLRU page which contains old XIDs, while
vacuum concurrently truncates the CLOG covering those XIDs. When the
backend then calls TransactionIdDidCommit() on those XIDs from the
local copy, you still get the error. The next commit will fix that
remaining race condition.

This was first reported by Sergey Zhuravlev in 2021, with many other
people hitting the same issue later. Thanks to:
- Alexandra Wang, Daniil Davydov, Andrei Varashen and Jacques Combrink
  for investigating and providing reproducable test cases,
- Matheus Alcantara and Arseniy Mukhin for review and earlier proposed
  patches to fix this,
- Álvaro Herrera and Masahiko Sawada for reviews,
- Yura Sokolov aka funny-falcon for the idea of marking transactions
  as committed in the notification queue, and
- Joel Jacobson for the final patch version. I hope I didn't forget
  anyone.

Backpatch to all supported versions. I believe the bug goes back all
the way to commit d1e027221d, which introduced the SLRU-based async
notification queue.

Discussion: https://www.postgresql.org/message-id/16961-25f29f95b3604a8a@postgresql.org
Discussion: https://www.postgresql.org/message-id/18804-bccbbde5e77a68c2@postgresql.org
Discussion: https://www.postgresql.org/message-id/CAK98qZ3wZLE-RZJN_Y%[email protected]
Backpatch-through: 14