Rationalize IConnection and ConnectionContext

[davidfowl]

This is the last remaining piece of work to do from #1453.

The cheapest solution I see is to:

• Implement ConnectionContext on the HttpConnection
• Introduce IConnectionFactory to the HubConnection layer

public IConnectionFactory
{
    Task<ConnectionContext> ConnectAsync(TransferFormat transferFormat);
}

The last thing to figure out after that would be how we do DisposeAsync (it could be Abort), see aspnet/KestrelHttpServer#2054

[davidfowl]

/cc @KPixel

[KPixel]

Great!
This looks like the last piece of the refactoring.

I haven't been able to work on this project this week. But I will have time for it next week.