TCP: Enhance Unseen Ack detection

The latest changes for ACKed unseen segments only partially improved their detection while introducing side-effects.

Reconciliation with maxseqtobeacked after loss events is too optmistically pushing forward when the receiver might not have acknowledged yet all the packets seen. We now rely on the unacked structure and better manage the maxseqtobeacked information in order to be more accurate.

See #8404 (closed) #11506 (closed) #18558 (closed) #18633