Function establish_connection 

async fn establish_connection<T: Transport + Send + Sync + 'static>(
    transport: &T,
    options: &ClientOptions,
    clean_start: bool,
    client: &mut Client<JournaledSession>,
) -> Result<EstablishedConnection<ReadHalf<T::Io>, WriteHalf<T::Io>>, DisconnectReason>

Perform TCP connect, send CONNECT, wait for CONNACK, and retransmit any pending QoS messages when resuming a session.