CRL future works

[JeyJeyGao]

As mentioned in #425, the following work items will be considered in the future:

3. Revocation mode control : Many users may prefer to use CRL due to network or OCSP server issues. Therefore, we need to provide a way to choose the preferred revocation solution rather than a fixed rule.
4. Revocation network request timeout threshold control, CRL cache TTL control, and CRL cache cleaning.
5. Delta CRL support