Evaluate back-pressure implementation for sending side WebSocket messages

[mliarakos]

Currently, the lagom.js WebSocket client does not support back-pressure for sending or receiving messages over the socket. As described here, back-pressure for receiving messages is not possible under the current WebSocket standard.

However, it is possible to provide back-pressure for sent messages using WebSocket.bufferedAmount, as described here. We should evaluate if sending side back-pressure can be implemented using this approach.