Optimize Await single Future

[MBkkt]

First of feel free to ask me(@MBkkt) any questions about this task, often the task may not be described in enough detail for devs who don't have the full context.

If we make Await single Future, we can do it without atomic reference counter in event

Check Wait function implementation:

We use OneShotCounter in case when single future, same possible for Await

Should be implemented here

Also will be great if you make this small task:

• Implement co_await operator for single yaclib::Future #194

[MBkkt]

Done