Flag previous calls as invalid

[JensGM]

This could possibly be done by scrambling the result hashes

    def invalidate(self, call, func=None):
        if not self.completed(call, func):
            return

        namespace = self.store / 'results' / call

        hash = func.hash if func is not None else namespace['latest']

        sha256 = hashlib.sha256(bytes.fromhex(hash))
        sha256.update(secrets.token_bytes(32))
        distorted = sha256.hexdigest()

        namespace[distorted] = namespace.pop(hash)
        if namespace['latest'] == hash:
            namespace['latest'] = distorted