Acknowledge sent packets when receiving a FIN

[carver]

Fix #103

Acknowledge sent packets using ack_num in FIN and DATA packets

More specifically:

The ack_nr on the received packet was previously ignored. In some scenarios, like with dropped packets or different utp implementations, that would cause the sender to wait indefinitely for the recipient to ACK state. But the recipient had already acked with the FIN (been ignored), and disconnected. Then the sender was waiting the full connection timeout to realize it.

Even worse, if more than just the final outbound FIN was acknowledged in the inbound FIN, the sender was considering the data transfer incomplete. This was causing errors in #81 .

With this commit:

• When the recipient acks the final FIN with a FIN, the sender doesn't get stuck waiting for the timeout, and returns from close() immediately.
• When the recipient acks some data with the FIN, the sender now returns from close() with an OK instead of Err.

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I tested this on a #81 locally, and (only) with this patch, I was able to get 10 successful test runs in a row, even when doubling the concurrency to 2000 connections.

[carver]

A more complete solution would probably be to move the ack handling into on_packet(). For example, the ack_num on a reverse-direction DATA packet is also currently ignored in the code. Putting it all in one place would reduce the code duplication, as well as slim down the number of function arguments to on_state() and on_fin().

[carver]

I'll rebase this on #109 when that's merged