[#165] Add cachecheck logging

[matthewhilton]

Closes #165
Closes #168

Changes:

• 2 new events:
  • cache_check: whenever the cachecheck check api is run.
  • cache_ping: whenever a new value is written to the cache
• Each one records details e.g. what the cached value was, what the db value was, and for cache_ping, what the value it just wrote to the cache was. It also logs more data e.g. site identifier and session redis prefix so it might be obvious if these are broken or not correct to the site.
• Settings to turn each of these on / off since they are debugging tools so don't always need to be turned on.
• Also a tiny bugfix for cachecheck cron to only run once every 8 hrs, instead of every min in the 8th hr

I'm still a bit undecided whether site identifier or session redis prexis is useful. For e.g. if you had a cache issue such as another site (site B) using the same prefix, these logs would be written on Site Bs logs but would have Site A's prefix. But if you only knew about site A, you would have no idea how to find site B.

Since event logs are logged, it could be possible to work this out if you grepped across all the site logs for a particular prefix and see what sites are putting those cachecheck/ping logs out.

TODO after merging:

• Cherry pick to MOODLE_39_STABLE -> [#165] Add cachecheck logging - MOODLE_39_STABLE cherry pick #167

Pull request checks

• I have checked the version numbers are correct as per the README

[matthewhilton]

FYI I also added a new part to the log where after it sets the cache, it immediately reads it back and logs this as well. This should help detect cache replication delays