Neighbor refresh fails for random neighbors with default base_reachable_time_ms

[joolli]

While testing the refresh timer code, some neighbors timed out and went NUD STALE if the base_reachable_time_ms was on the default setting of 30000.

We need to adjust the timer or adjust the formula so that some neighbors don't fall through even with short timers or change the base_reachable_time_ms or issue a warning when the time is set too low.