Clarify ssh connection state concurrency issue

[gaetanzanella]

let connection = SSHConnection(...)
connection.stateUpdateHandler = { state in
    if state != connection.state {
        // ????
    }
}

This case can happen as the update callback is called asynchronously. Is it correct? We should check how NWConnection manages this case.

[gaetanzanella]

https://github.com/swift-server/async-http-client 👀