Caching CMM max age should be required to be greater than 0

The caching CMM should require that `max_age` is greater than 0.

If initialized with a `max_age` value that is not greater than 0, we should raise a `ValueError` with an appropriate error message similar to the checks we already apply:

https://github.com/aws/aws-encryption-sdk-python/blob/master/src/aws_encryption_sdk/materials_managers/caching.py#L98-L109