Use UnauthedEthStream pattern for creating an EthStream

[Rjected]

A P2PStream is created by first constructing an UnauthedP2PStream (which wraps an underlying Stream + Sink) and awaiting on the UnauthedP2PStream::handshake method, returning a P2PStream:

let unauthed_stream = UnauthedP2PStream::new(stream);
let p2p_stream = unauthed_stream.handshake(client_hello).await.unwrap();

This ensures that all P2PStreams in use are with peers that have successfully completed the p2p handshake.

EthStream currently uses an authed flag to determine if a stream is authenticated, when instead we can take the above approach to construct an EthStream. Similar to #157, we should also return the incoming Status with the authenticated EthStream.