Unexport LocalConnection, favor an e.g. Overlayer interface

[peterbourgon]

From #33,

It looks like LocalConnection would not need to exported either if it implemented an interface with an OverlayConn method, so the likes of https://github.com/weaveworks/weave/blob/master/router/network_router.go#L189 can use that instead of a cast to LocalConnection.