Support processing network response body notifications on background threads

[jdm]

This is a feature that Gecko supports - for responses where nothing useful can be done until the full response body is present, there's no reason to queue an event in the main thread that just pushes data to a buffer. If the buffer is threadsafe, that pushing can happen in a background thread while the main thread works on more important things. We should make it easy to opt-in to this optimization.

[nox]

@jdm Does the buffer even needs to be threadsafe? Can't we just send it to main thread once all the data has been accumulated?

[jdm]

That's a good point!