The mock is used in few tests, implements model.ServiceDiscovery.

1. Should be next to the other registries.
2. Needs a couple of /debug/registry... http handlers, to allow a test program to modify the registry
3. Should be called 'memory' (or similar) - it is not a mock.

We also need a helper to populate the memory registry and cause 'invalidation' events, to simplify
testing the EDS/CDS updates and make them more deterministic.