Support databases as storage

[kebugcheckex]

This is a feature request. I will probably work on it later.

Current storage uses file system to store keys and certificates. This will not scale well when the number of certificates grows to a magnitude of 10^4 or larger. I'm considering create a storage adapter that can support databases, both relational databases (e.g. MySQL) and key-value storage (e.g. Redis), while retaining the ability to use file system as the storage.

I don't have a detailed design yet. On the high level, we can abstract the load/save operations and have different implementations behind the scene. I will add more details soon.