"Replace" for core cache API

[cceckman-at-fastly]

#473 has the last bits of the core cache API that we expect to deliver for now. The simple cache API should all work. The core cache API is partly implemented:

• Non-transactional insert and lookup are implemented
• Transaction is implemented
• replace is not implemented

The replace entry point and associated calls turn up an error if called, so this should be a clear error.

If you're interested in an implementation of replace, give a shout here and that can inform Fastly's efforts.