WIP: SSU: Implement fast retransmit
For review only. Untested. This does NOT change the window size strategy; it preserves the current behavior of advancing the window for all new acks, even if there are gaps behind. Adds sequence number to OMF Stores recently-acked message and seq. numbers in a LinkedHashMap Calculate the highest-acked seq. number for every incoming packet. Marks messages older than highest acked as nacked Fast-retransmits after 3 nacks TODO: Syncing, cleanup TODO: Reduce MAX_RESEND_ACKS and RESEND_ACK_TIMEOUT ?