NAKACK: maintaining stats ist costly

In NAKACK, we maintain mulitple hashmaps of statistics data, e.g. average retransmission times etc, if stats="true" (default). This is costly, and some stats updates are on the critical path.

Investigate whether we can apply lock narrowing for stats. Possibly use more concurrent data structures. Maybe also turn unused stats off, e.g. if we don't do retransmission based on avg retransmission times, turn retransmission stats off.

This could be moved into 2.9 if there is not enough time to do this in 2.8