Fix: Stop retransmitting third ack after fourth ack without data-ack

RFC 6824 says that any fourth ack should stop the retransmission of the
third ack with MP_JOIN.

This patch fixes it, as previously we only stopped retransmitting if the
fourth ack had a DATA_ACK.

Reported-by: Varun Taneja <sunvarun@gmail.com>
Signed-off-by: Christoph Paasch <christoph.paasch@uclouvain.be>
Fix further infinite mapping

Signed-off-by: Christoph Paasch <christoph.paasch@uclouvain.be>
Cleanup

Signed-off-by: Christoph Paasch <christoph.paasch@uclouvain.be>