Outage is always adding some small db overhead on each page bootstrap

[brendanheywood]

https://wrms.catalyst.net.nz/wr.php?request_id=345128

Whatever config we need, consider adding this to the main core config instead of plugin config to avoid a second muc / db call during bootstrap

https://github.com/catalyst/moodle-auth_outage/blob/master/classes/dml/outagedb.php#L214-L224

A simple approach:

• [ ] cache the next outage if any
• [ ] on any outage mutation reset the cache
• [ ] BUT we want zero extra calls to be as lightweight as possible. So serialize this into as light weight as possible (which is possibly just a timestamp of the next warning period) and put this into a core config item which means it is loaded for us during bootstrap. All we care about is if we are after this timestamp or not. If we are close to this time stamp then we can make a db call if needed to get more info.