Throttle RPC and WS concurrent active clients

This change set implements tuneable concurrent active clients throttling.
I don't believe you need this function at all.  Simply change the signature on the `throttled` function from `func throttled(threshold int64, h http.Handler) http.Handler {` to `func throttled(threshold int64, f http.HandlerFunc) http.Handler {` and then change `h.ServerHTTP(w, r)` to `f(w, r)`.

There is no need for another layer of indirection.