IPv4Defragmenter DiscardOlderThan returns wrong number.

According to documentation, DiscardOlderThan() returns the number of FragmentList aka number of fragment packets it has discarded. The actual return value is the total number of the ip flows the derangementer holds in it's flows map, including the flows that was defragmented successfully and was flushed.

Possible solution is deleting an ip flow from the flows map when flushing it, instead of replacing it with a new ip flow.