Use native backend functionality to delete expired responses

[alessio-locatelli]

TO-DO

• MongoDB
  - How to implement: https://www.mongodb.com/docs/manual/tutorial/expire-data/
  - Implemented in requests-cache: yes.
• Redis
  - How to implement: Create a new hash with response_key: TTL and use a Lua server-side script to discard an expired response by the key. Use HEXPIRE.
  - Implemented in requests-cache: no.
• Sqlite: ?
• Filesystem?

Why

• Not necessary to run manually delete_expired_responses() if a backend supports TTL.
• Performance: no redundant network I/O if we only get a value to check that it has not expired.

Pull Request

(a draft) alessio-locatelli#1