fix handling of cached segments in ThroughputHistory

Created by: spiterikevin

#2003 changed the behavior at startup when only cached segments are available to calculate throughput. When no uncached segments are available, a throughput estimate is taken from cached segments. This is not ideal, but ABR rules need something to go by. As from #2003, the cached-fragment throughput is incorrectly divided by a factor of 1000. This PR fixes that issue.