Inconsistent typing in RPC client

Right now we should do something like this:
```go
for m := range ch {
if m["Event"] == "query" {
fmt.Println("Query payload: ", string(m["Payload"].([]byte)))
c.Respond(uint64(m["ID"].(int64)), []byte("response"))
}
}
```
due to inconsistent typing (``uint64`` and ``int64``).

And yeah, I don't understand why use maps here instead of structs as they are already defined in serf/serf. Godocs are silent about this, I was only able to obtain the map format via testing it myself.