Add flag to disable synchronous IO

[halter73]

This is the equivalent of aspnet/KestrelHttpServer#1521.

Both servers will need to throw an InvalidOperationException on Request.Body.Read() and Response.Body.Write(). You can opt back in to synchronous IO using IHttpBodyControlFeature.AllowSynchronousIO.

[Tratcher]

@muratg This needs to go into 2.0 or else it's a major breaking change.

[muratg]

Yeah, we decided to do this in Kestrel as well. Let's do it in 2.0.0.