Use ConfigureAwait(false) throughout RHost & RSession code

[int19h]

Use ConfigureAwait(false) throughout RHost & RSession code to avoid unnecessary thread affinity, and prevent deadlocks when async methods are called synchronously from the UI thread.

[int19h]

Please hit Merge if it looks okay.

[AlexanderSher]

ConfigureAwait(true) posts continuation after await into SyncContext if its type is different from base SynchronizationContext type, or into TaskScheduler if is different from TaskScheduler.Default. Otherwise it is scheduled into default TaskScheduler.
ConfigureAwait(false) always schedules continuation after await into default TaskScheduler.

Specifying ConfigureAwait doesn't guarantee that UI thread will not be blocked:

private async Task M1() {
    await Task.Yield();
    while (!_tcs.Task.IsCompleted) { }
}

 // Blocks UI thread
private async void Button_OnClick(object sender, RoutedEventArgs e) {
    // Here we are in UI thread
    await M1().ConfigureAwait(false);
    // Here we are in background ThreadPool thread
}

 // Doesn't block UI thread
private async void Button_OnClick(object sender, RoutedEventArgs e) {
    // Here we are in UI thread
    await Task.Run(() => M1()).ConfigureAwait(true);
    // Here we are in UI thread
}

I'll revert this change and add Task.Run call to RSessionEvaluation.EvaluateAsync to insure that it will not freeze UI if called from UI thread