Improving kv test coverage

* Adds unit tests to kv/local_kv.go and kv/dist_kv.go
* Generalizes gossip network simulation so that it can be used in tests which require gossip networks
* The unit tests for kv/dist_kv found a user permission bug which led to false positives. This is fixed in storage/prefix.go.

I'm brand new to Go, so I would appreciate any advice!
This is clever using the gossip simulation network this way. Cool--happy to see the gossip simulation code make it into more of the testing pipeline so it doesn't rot.