The rule is now: take the value from the CircuitPriorityHalflife
config option if it is set.  If it zero, disable the cell_ewma
algorithm.  If it is set, use it to calculate the scaling factor.
If it is not set, look for a CircPriorityHalflifeMsec parameter in the
consensus networkstatus.  If *that* is zero, then disable the cell_ewma
algorithm; if it is set, use it to calculate the scaling factor.
If it is not set at all, disable the algorithm.