The RTTStats are used by the logging package. In order to instrument the
congestion package, the RTTStats can't be part of that package any more
(to avoid an import loop).
Duplicate detection uses the same data structure that is used to track
received packets to generate ACK frames. That means that after an old
ACK range has been pruned, a severly delayed packet might be incorrectly
detected as a duplicate.
As we wouldn't have acknowledged receipt of this packet, this situation
would have resulted in a retransmission by the peer anyway, so dropping
the packet won't cause a big regression.
