implement key-value caching

[ecton]

While browsing the Redis site, I noticed this page detailing a new feature in Redis 6 for automatic client caching.

I personally think that their approach is a bit heavy-handed, although it does allow for easy adoption in existing applications. For BonsaiDb, I think a utility caching type that sits atop a KeyValue implementor would be pretty straightforward to implement and could provide a caching-specific API.

• Blocked by Add notification of changed keys #155