fix: free resources in async server example

[jmar910]

I have been playing around with this library to learn zig, and I think I found an issue in the async example where clients are not being cleaned up correctly. I believe the client connection needs to be closed and the client struct needs to be freed when the loop breaks and handle returns (i.e. client is disconnected)

I am new to zig so I could be way off base here, but let me know if these changes make sense. Since this is just™️ an example, I figured it would be a pretty low stakes way to test my understanding and contribute.

[ikskuh]

I think we can delete the async example as it won't be compiled anyways.

async is being removed from the language and replaced with lower level primitives