- Divorce the allocation of free request slots and the congestion
    control. Make the congestion control apply only to when we
    actually send data over the wire. This means that we *do* apply
    congestion control to resent requests: if a timeout has occured,
    and there are too many requests on the wire, delay resending until
    the congestion algorithm allows it.

  - Improve spinlocking by putting the congestion avoidance algoritm
    under xprt->sock_lock. This lock has to be taken *anyway* in
    (almost) all cases where we are updating the congestion control
    data.