validate

The contract of this function is that it returns a ChallengeValidationResult.Success iff a valid challenge matching the passend csr from the client is found. In all other cases, it must return a ChallengeValidationResult.Failure:

• It must return a ChallengeValidationResult.Failure.NonceExtraction if nonce extraction fails (relevant for nonce-cache based implementations)

• It must return a ChallengeValidationResult.Failure.Other if other validation errors occur, such as no valid challenge matching the passed csr. In addition, it should also remove all expired challenges, to keep stale challenges from inflating memory/storage.