Add recv stream id so stream can be tracked during callbacks

[gfodor]

Right now there doesn't seem to be a way to identify where incoming bytes came from in terms of the original stream. (You can only see the connection, and the stream struct lacks identifying information.) This adds the recv_stream id to the struct.

[gfodor]

Actually this PR can probably be closed - I did not understand there was one GenServer per stream. That said, the integer id for the stream does seem to be useful information in general so maybe worth merging.

[bugnano]

My intended way for using the stream was that if you need an id, that id can be self() in the handle_stream callback, and you do something like:

def handle_stream(%Stream{} = _stream, state) do
  state = Map.put(state, :recv_stream_id, self())
  {:continue, state}
end

without the need of patching this library.

Consider that state is a variable that is available to the user to do anything he wants.

Sounds good?