pgsql: Use larger segment file names for pg_notify

Use larger segment file names for pg_notify

This avoids the wraparound in async.c and removes the corresponding code
complexity. The maximum amount of allocated SLRU pages for NOTIFY / LISTEN
queue is now determined by the max_notify_queue_pages GUC. The default
value is 1048576. It allows to consume up to 8 GB of disk space which is
exactly the limit we had previously.

Author: Maxim Orlov, Aleksander Alekseev, Alexander Korotkov, Teodor Sigaev
Author: Nikita Glukhov, Pavel Borisov, Yura Sokolov
Reviewed-by: Jacob Champion, Heikki Linnakangas, Alexander Korotkov
Reviewed-by: Japin Li, Pavel Borisov, Tom Lane, Peter Eisentraut, Andres Freund
Reviewed-by: Andrey Borodin, Dilip Kumar, Aleksander Alekseev
Discussion: https://postgr.es/m/CACG%3DezZe1NQSCnfHOr78AtAZxJZeCvxrts0ygrxYwe%3DpyyjVWA%40mail.gmail.com
Discussion: https://postgr.es/m/CAJ7c6TPDOYBYrnCAeyndkBktO0WG2xSdYduTF0nxq%2BvfkmTF5Q%40mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/2cdf131c46e631addfc386f6106e52a1b8cc3a70

Modified Files
--------------
doc/src/sgml/config.sgml | 16 ++++
doc/src/sgml/ref/listen.sgml | 1 +
doc/src/sgml/ref/notify.sgml | 1 +
src/backend/commands/async.c | 122 +++++++-------------------
src/backend/utils/misc/guc_tables.c | 10 +++
src/backend/utils/misc/postgresql.conf.sample | 3 +
src/include/commands/async.h | 1 +
7 files changed, 62 insertions(+), 92 deletions(-)