Throttle RPC and WS concurrent active clients

This change set implements tuneable concurrent active clients throttling.
My previous comment on this got eaten by Github "outdated diff".

Using an http.Handler interface instead of an http.HandlerFunc is more generic and allows all implementors of the http.Handler interface to use throttled. This might be premature abstraction but if you want to reuse this function in other places, this is the recommended way to do it.

EDIT: I added a convenience throttleFn which wraps throttle with the intended purposes.