Proper Handling of Caching

[mikkokotila]

It seems that roughly speaking we have:

• Application caching
  • [ ] Configuring application caching #150
  • [ ] Purging application caching (this should not require any work)
• Server caching
  • [ ] Configuring server caching #152
  • [ ] Purging server caching @pdey
• Edge caching
  • [x] Configuring edge caching
  • [x] Purging edge caching #100

For the sake of completeness, there is also browser caching, but that is not something we can directly control.