Client-side caching and local control, to support servers with no cache support

[jbfaden]

The Java client should provide optional caching independent of proper caching where if-modified-since is sent to the server. The client should be able to control what is used and kept, for example, to allow the clients to repeatedly use the data for the next hour.

This should be coordinated with the Python client, so that the two can use a common cache.

[jbfaden]

See https://github.com/hapi-server/cache-tools