Drastically simplify Crispin code.

DummyCrispinClient was a hairbrained idea to save all the raw bits we
pulled off the wire to disk and be able to replay operations in a sort
of offline mode, for testing and things. In retrospect, this is not the
most straightforward way to get the benefits that we want, namely the
ability to test message parsing separately from message download, and
the caching schema was complicated enough that we never ended up
finishing it or using it for anything.

Let's rip it out and move on. As it is, it's just confusing.