retry interval in milliseconds for redis session cache is far too high

Setup

1. The prerequisite is a working Redis installation available on localhost.
   1. Install the PHP Redis extension appropriate for your PHP version. e.g. for PHP 7.4 on Ubuntu

      sudo apt install php7.4-redis

   2. Run a Redis server via Docker

      docker run --name my-redis --network host -d redis

2. Add the following to config.php

   define('TEST_SESSION_REDIS_HOST', '127.0.0.1');

   $CFG->session_handler_class = '\core\session\redis';

   $CFG->session_redis_host = '127.0.0.1';

   $CFG->session_redis_acquire_lock_timeout = 120;

   $CFG->session_redis_lock_expire = 7200;

Test

1. Set up PHPUnit and run:
   vendor/bin/phpunit lib/tests/session_redis_test.php
   to verify that there are no regressions.
2. Using the redis-cli set the connection timeout to 5 seconds.
   CONFIG SET timeout 5
3. Then download the redis_sessions_timeout.php script from this issue and put it in the webroot and run it as the apache user:
   sudo -u www-data php path/to/moodle/redis_sessions_timeout.php

Before the patch, this should wait a very long time, after applying the patch it should be rather quick to reconnect to the Redis server.