[inetstack] Retransmit Timer Issues

[BrianZill]

We should review the retransmit timer implementation, specifically for the following issues:

• The code seems overzealous about pushing the retransmit timer into the future. One case in particular is in the retransmit function, where it resets the expiration deadline regardless of why retransmit was called (it can be called not only on timer expiration, but also because of Fast Retransmit -- where I believe the deadline should remain untouched).
• The timer appears to also be extended upon every arrival of a new ACK, is this really desired behavior?
• The timer doesn't record the sequence number it is tracking for retransmission, instead it does the above, which could make it wait longer than desired for retransmission. Again, review that this is desired behavior.