Enforce buffer limits after Pipe behavior change

[BrennanConroy]

We need to react to dotnet/corefx#35711 which removes the old behavior of throwing when the writer is waiting because of backpressure and the reader examines everything but doesn't consume. Instead the reader will now unblock the current write allowing more data to be buffered.

We'll need to enforce our own limits in the HubConnectionHandler.DispatchMessagesAsync() method.