Improved await support of ReactiveProperty, ReactiveCommand

[neuecc]

I've fully refactored await support of ReactiveProperty and ReactiveCommand.
It can handle CancellationToken correctly and add GetAsyncHandler that can reuse(zero-allocate) await event.
Changed push event to awaiter from internal to external.

Breaking Change: AsyncReactiveCommand does not support await.

how to use from awaitable.md

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Awaitable

You can use await operator to ReactiveProperty(includes Slim), ReadOnlyReactiveProperty(includes Slim), ReactiveCommand.
When use await operator to them, then wait a next value will be published.

For example:

// View with CancellationTokenSource
public partial class SampleWindow : Window
{
    CancellationTokenSource cts;
    SampleViewModel viewModel;

    public SampleWindow()
    {
        InitializeComponent();
        cts = new CancellationTokenSource();
        viewModel = new SampleViewModel(cts.Token);
    }

    protected override void OnClosed(EventArgs e)
    {
        // on finish, cancel all await.
        cts.Cancel();
        cts.Dispose();

        base.OnClosed(e);
    }
}

// ViewModel with CancellationToken
public class SampleViewModel
{
    public ReactiveCommand MyCommand { get; private set; }
    public ReactiveProperty<int> ClickCount { get; private set; }

    public SampleViewModel(CancellationToken closeToken)
    {
        MyCommand = new ReactiveCommand();
        ClickCount = new ReactiveProperty<int>();

        // handling event by async/await.
        SubscribeAsync(closeToken);
    }

    async void SubscribeAsync(CancellationToken closeToken)
    {
        using (var handler = MyCommand.GetAsyncHandler(closeToken))
        {
            while (true)
            {
                await handler; // await when clicked.
                ClickCount.Value += 1;
            }
        }
    }
}

If you await multiple times, you should get ObservableAsyncHandler<T> from GetAsyncHandler. it can await multiple times on zero allocation. If you await single time, you can use await command.WaitUntilValueChangedAsync(token).

Note: you can await ReactiveProperty directly but we recommend use GetAsyncHandler(multiple) or WaitUntilValueChangedAsync(one shot) with pass over CancellationToken.