Several seconds settings became time.Duration with no upper bound, so
a large value wrapped negative or to zero. maxTokenAgeSeconds then
disabled the bearer age check, and a zero health check interval
panicked time.NewTicker. A circuit breaker threshold above MaxInt32
narrowed to a value that tripped on the first failure. A huge poolSize
allocated the connection channel up front. Validate now rejects these
values. Sane and default values are unchanged.
