Change KestrelServerOptions.AllowSynchronousIO default to false

[halter73]

KestrelServerOptions.AllowSynchronousIO is going to default to true for 2.0.0. Hopefully we can change this to false in our next major release.

aspnet/Announcements#252 (comment)

@muratg @Tratcher We'll probably want to file a similar issue for HttpSysServer.

[muratg]

@Tratcher could you file a bug on HttpSysServer to track this for 3.0.0?

[Tratcher]

https://github.com/aspnet/HttpSysServer/issues/373

[davidfowl]

We need to do this again in 3.0 https://github.com/dotnet/corefx/issues/32665

[davidfowl]

@halter73 @muratg @Tratcher we should do this earlier rather than later.

[Tratcher]

I just fixed response compression so this is unblocked.

[Tratcher]

So System.IO.StreamReader.EndOfStream calls sync Stream.Read...

>	Microsoft.AspNetCore.Server.IIS.dll!Microsoft.AspNetCore.Server.IIS.Core.HttpRequestStream.Read(byte[] buffer, int offset, int count) Line 30	C#
 	Microsoft.AspNetCore.Server.IIS.dll!Microsoft.AspNetCore.Server.IIS.Core.WrappingStream.Read(byte[] buffer, int offset, int count) Line 66	C#
 	System.Private.CoreLib.dll!System.IO.StreamReader.ReadBuffer() Line 618	C#
 	System.Private.CoreLib.dll!System.IO.StreamReader.EndOfStream.get() Line 304	C#
 	InProcessWebSite.dll!TestSite.Startup.ReadAndWriteEchoLines(Microsoft.AspNetCore.Http.HttpContext ctx) Line 351	C#