ACK Trimming is too aggressive

[huitema]

When debugging the test "app_limited_rpr", I observed that lots of packets are declared lost at the server when they have in fact been received at the sender. This is due to ACK loss. Multiple Data packets are also lost, which means the ACK will have to contain several ranges. The number of ranges is trimmed after they are sent more than 4 times, but more than 4 consecutive ACKs are lost in the test. When an ACK finally arrives, all the packets in the "trimmed" ranges are lost, which causes a number of unnecessary retransmissions.