Add ability to create Connection from existing socket

[a1ien]

This will be useful for example if we create socket in another process and use technique like snedmsg to pass socket descriptor between process.
The expected Api probably like new_connection_from_socket<S>(socket: S) where S: sys::AsyncSocket ...
What you think about this idea?

The use case for this feature. We have application that live in different namespace. And we can create netlink socket in different namespace and pass created socket to another application.