Add support for non-mainnet request cache validation thresholds:

- If non-mainnet ethereum, allow the use of an integer value for the
  request cache validation threshold. Pre-configure "safe" default
  values for some common non-mainnet chain ids based on their
  varied finality mechanisms.

- This integer value represents the number of seconds from time.now()
  that the request cache deems as a safe enough time window to allow
  the request to be cached.

- Update the tests to reflect these changes.

Bonus:

- Add a note to the ``request_mocker`` to make sure that mocked results
  are of the correct expected types based on JSON-RPC spec (e.g. hex
  strings instead of ints for numbers, etc.)