Lift circuit_build_times_disabled out of circuit_expire_building loop
profiling shows this function occupies about 2% of CPU time on an active relay because networkstatus_get_param uses string comparisons. I think this change is safe because new consensuses are not fetched in this function, nor can config options be modified while in this loop, nor is the state file written to. Moreover, the worst case scenario is that a warning is accidentally shown or not shown.