What was wrong?

closes #3506

How was it fixed?

• 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.
• Added comprehensive testing around the mechanism and all affected endpoints.

Unrelated:

• 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.)

Todo:

• Add or update documentation related to these changes. This documentation should detail the underlying mechanism of how (and why) to check certain endpoints against a validation threshold so that users know:

  1. When to reach for caching in the first place
  2. If they reach for caching, clarify internal calls can be made to get the necessary validation data
  3. Know how they can tweak the validation threshold to lower the number of internal calls made (or set it to None to cache any and all requests)

• Add entry to the release notes

• Clean up commit history

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