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!
Now that we are using this in tests, it would be better to move this line into main() below.