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!
I actually added all the methods in readOnly, read/write, and writeOnly categories to all of the tests. It didn't seem to be a time-limited factor, but I don't have to do it this way.