Fake client/server pair implementing proposed GBS Python interface (part 1: immutables)

[itamarst]

In order to refactor the Tahoe-LAFS client code to support two protocols in parallel, we need some implementation of a new low-level client storage Python interface (let's call it IStorageClient). The production IStorageClient implementation will talk the GBS HTTP protocol to the storage server.

However, since the wire protocol is going to have to be audited, and since having a (verified) fake implementation is useful for writing more isolated tests, a useful first step would be a IStorageClient implementation that is implemented in-process.

The deliverables for this will be:

1. A new interface, IStorageClient, corresponding the the proposed HTTP protocol.
2. A compliance test suite for IStorageClient providers.
3. A (verified) fake IStorageClient. Since it won't talk over the network, there is no need for a separate server.

It will be limited to immutables, with mutables in a follow-up ticket.