Support streaming request / response body

At the moment SW proxy does forward `ArrayBuffer` for request / response bodies. Which seems to work in use cases currently being proved, however it won't be able to handle things like video streams etc...

Ideally `ReadeableStream` would have being made transferable, but sadly they are not (, at least yet). In order to support this we need to polyfill that via `MessagePort` API. We could either create one per request / responce and send it over with response or alternatively multiplex via same `MessagePort`. Former has benefit of not needing to keep track of request IDs, however later might have non-negligible impact on memory use.

Start by tracing the service-worker proxy's request and response body handling, then review how the MessagePort API could carry the stream. The issue leaves the per-request versus multiplexed design unresolved, so determine that approach before implementation. Done means streaming request and response bodies, including video streams, work through the proxy.

Written by the indexing model from the issue text.