Development: Updates to grade caching and utility scripts.

[mckeownp]

Updates to the grade caching to make it work better and to make it play nice with Moodle 5.0.

• The scheduled cachecleaner task now only runs if the grade cache is enabled and is now explicitly only for a file store cache backend.
• The Redis cache cleaner can be used if cleaning is needed for Redis.
• The gradecache is still experimental and hasn't been fully tested on Redis. So, if you're going to use it, we recommend sticking to a file store backend for it.

Updates to other utility scripts (eg, bulktester, cachepurger, etc) also updated to be even more handy (and Moodle 5.0 friendly).