Function create_common 

async fn create_common<RT: Runtime>(
    chan: Arc<Channel>,
    timeouts: Arc<dyn TimeoutEstimator>,
    rt: &RT,
) -> Result<PendingClientTunnel>

Try to make a PendingClientTunnel to a given relay, and start its reactor.

This is common code, shared by all the first-hop functions in the implementation of Buildable for ClientTunnel.