PeerState.java: rework ACK resend logic

[jogger]

The current ACK resend logic is a leftover from freenet. Randomised resends over 5 minutes help nothing if we want the resend to prevent a 1000 ms timeout on the other end. Any reasonable assumptions about failure rates lead to the choice between 0 and 1 resends. A single resend costs 4 bytes and possibly prevents a retransmit so this seems to be a good choice for typical error rates around 1%.
So I propose to do a single resend with a gap of ACK_FRQUENCY, which will in almost all cases be fast enough to prevent a possible retransmission and avoid being hit by two consecutive packets failing.

[zzz]

Please point us to the file and line number or method in question

[jogger]

This aims at reducing the duplicates received to near zero, as a double ACK should get through at any reasonable rate of packet loss. A quick hack revealed this is in fact possible.

Needs a single unified data structure for all full ACKs noting a send time.

Thus needs to rework nearly all ACK methods in PeerState? as well as modifications to packet builder and ACKsender.