[beam-lib] Invalid receiver handeling

[Threated]

• Explicitly check for invalid receivers
• Fix tests

[TKussel]

I have the feeling, that the handle_invalid_receivers() function can be elegantly replaced by the type system. All my initial ideas would lead to ugly generics, but maybe you have a better idea for a later refactor.

[Threated]

I wanted to create an Extension trait for request which would be very elegant but because async fns in traits are not stable yet I just made it a function. I did not want beam_lib to depend on async_trait but I could maybe create my own future.

[Threated]

Wasn't even that difficult. Its very clean now. Although I hope async traits get stabilized soon.