UNICAST: switch from AckSenderWindow/AckReceiverWindow to Table<Message>

UNICAST:

• The sender has an AckSenderWindow, the receiver an AckReceiverWindow
• The sender keeps retransmitting until the receiver delivers the message and acks it

Suggestions:

• Similar to NAKACK2, don't use a separate retransmitter for every member (window), but have just 1 retransmitter task scan the message tables and do retransmission
• The sender's AckSenderWindow can also be replaced with a Table<Message>
• Tthe receiver window can be replaced with a Table<Message> as well !

If we can do this, then we have

• only 1 proven data structure (Table<Message>)
• not 1 retransmitter per window, but 1 for all the windows (saving space) !