Improve caching

[AaronAcerboni]

Improve caching

[premasagar]

It's worth taking a look at https://github.com/dharmafly/elsewhere to see how caching is done there these days.

[AaronAcerboni]

• Truncate cache, according to a limit. Keep a total amount of cached items at one time 👌
  Cache size is kept to a specified limit ('lib/config.js') and oldest cached items are cleared to make way for newer entries.
• Cache time for considering certain items to be in need of an update. eg 1 hour 👌
  Cached items stay for a specified time limit ('lib/config.js') and then are cleared.
• If request fails, serve cached data 👌
  Cached items are prioritized to the client before any requests to other sites have to be made. In addition failed requests do not get cached.
• Purge time, eg 1 week 👌
  All cached items are now cleared on a weekly basis by default. This option can be changed or turned off ('lib/config.js').