[autothrottle] auto-flush cached rates on interval

[jamiealquiza]

Autothrottle should flush cached throttle rates on interval to reconcile out of sync configurations that may have been set externally. One issue with flushing the rates blindly is that calculating new rates accounts for the most recently set rate; period flushing would cause the throughput to drop until the next interval.

It may be worth adding some other parameter that causes autothrottle to simply ignore the change threshold parameter rather than flushing the rates.