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!
Don't capitalize error strings (usually). See https://code.google.com/p/go-wiki/wiki/CodeReviewComments#Error_Strings.