Replace util/io with tokio

Currently, the network uses util/io.
It returns a token on an event, instead of the related stream. So the network module should manage the relationship between the token and the stream.
If the network module uses tokio, this complexity can be removed.

Start by locating util/io and the network module, then trace how events currently connect tokens to streams. Review the tokio integration point and determine the full scope of the replacement. Done means the network module uses tokio and no longer needs to manage the token-to-stream relationship.

Scritto dal modello di indicizzazione a partire dal testo della issue.