Design and implement support for Hub methods returning Streams and IObservable<T>

[analogrelay]

We want to support Hub methods returning streams of data, like this:

public IObservable<Thingy> Subscribe()
{
}

public Stream DownloadFile()
{
}

We need a design and implementation to support this.

[analogrelay]

Related, but not covered here: Sending binary data to the Hub. We can do this using byte[] right now, we just have to make it as efficient as possible.

[ReubenBond]

As a little proof of concept, not quite related to SignalR, but which I made because SignalR didn't support it: WebStreams, eg: https://github.com/WebStreams/WebStreams.Samples/blob/master/StockTickerController.cs

And methods can also consume observables: https://github.com/WebStreams/WebStreams.Samples/blob/master/ChatRoomController.cs#L48
I used Orleans as the "backplane", but that's not in any of those repos.

[davidfowl]

Nice!