etcdserver: expose store statistics

Easy part of #1278
I see. I was taking a slightly different approach with the others where the `httpserver` package does the marshalling, because this is more consistent with the other endpoints (for example, `EtcdServer.Do` returns `Response`s, not JSON). But stats are arguably a slightly special case. Let me think about it and see how it ends up looking in my self/leader stats patch.