Add expiration to cache entry

[madsodgaard]

• Adds expiration functionality recently added to the Vapor Cache to prevent the cache being filled up with stale values, such as old rate-limiting keys.
• Allows customization of the error thrown if they client is sending too many requests:

req.gatekeeper.gatekeeper(on: req, throwing: MyError())

GatekeeperMiddleware(error: MyError())

• Fixes an issue where effectively even if the config was 5 requests per second, the client would only really have 4 request available and be rate-limited on the 5th. They are now correctly first rate-limited on request no. 6.

[siemensikkema]

Looks good! Just a small remark. I’ll pre-approve :)