Add async methods and examples

[circler3]

I add async methods and examples for those who want to use it in async scenarios.

[Vect0rZ]

@circler3 Thank you for your contribution. The change is generally safe to merge, but I will need some time to give it a test and review it locally. If everything is alright, will merge it.
One thing I noticed is that the only frame processing method that is async is OnStreamFrameAsync and we might want to update the other ones. This might pollute the QuicConnection class, but I think it's okay in the general case.

Thanks!

[GF-Huang]

Any progress?

[laolarou726]

@Vect0rZ Any progress for this?

[AntiTenzor]

The library has very basic old-school coding style.
It is a great advantage.

Please, avoid async(s) at all costs.

Also it looks 'fresh and fashion' actually it has a lot of issues
including performance issues and exception handling.

Nice and clean API we see in this implementation is a great treasure.