Using nonblocking task completion source hack for Source.Queue

[Horusiath]

When we Source.Queue<> method, it can be materialized into queue with OfferAsync method. Problem is that, awaiting for it may cause a deadlock - it's a classical problem with awaiting on the TaskCompletionSource<>.Task and using SetResult on that task in the same execution context. We already met it before.

This issue is easy to fix in .NET Standard 2.0 by using special option when creating TaskCompletionSource<>. For .NET Standard < 1.6 we already provided a hack around it (which uses Task.Run to escape from execution context): it's suboptimal, but it's used only for older framework versions.

This PR uses our fixer method set to overcome this deadlock issue.

Example, I've used to recreate this issue, was a Akka.Streams TCP server:

// Server
var connectionHandler =
    Framing.SimpleFramingProtocol(128)
        .Reversed()
        .Join(Flow.Identity<ByteString>()
            .Select(bytes =>
            {
                var text = bytes.ToString(Encoding.UTF8);
                return ByteString.FromString("ECHO" + text);
            }));

var binding =
    await actorSystem.TcpStream()
        .Bind("localhost", 7000)
        .To(Sink.ForEach<Akka.Streams.Dsl.Tcp.IncomingConnection>(connection =>
        {
            connection.Flow.Join(connectionHandler).Run(materializer);
        }))
        .Run(materializer);

Console.WriteLine($"Server listening on {binding.LocalAddress}. Press Enter to stop");
Console.ReadLine();

// close the server
await binding.Unbind();

And client:

// Client
var input = Source.Queue<string>(100, OverflowStrategy.Backpressure)
    .Select(str =>
    {
        return ByteString.FromString(str);
    });

var output = Flow.Identity<ByteString>()
    .Select(bs => bs.ToString(Encoding.UTF8))
    .To(Sink.ForEach<string>(Console.WriteLine));

var connectionDef = actorSystem.TcpStream()
    .OutgoingConnection(new IPEndPoint(IPAddress.IPv6Loopback, 7000))
    .JoinMaterialized(Framing.SimpleFramingProtocol(128).Reversed(), Keep.Left);

var client = input
    .Log("stdin")
    .ToMaterialized(connectionDef.ToMaterialized(output, Keep.Left), Keep.Both);

var (queue, connectionTask) = client.Run(materializer);

var connection = await connectionTask;

Console.WriteLine($"Connected to: {connection.RemoteAddress}. Start typing:");

while (true)
{
    var data = Console.ReadLine();
    await queue.OfferAsync(data).ConfigureAwait(false); // we'll deadlock here
}

[Aaronontheweb]

LGTM