The function ReceivedAck contained some if/else switches to manage ACK
which, by definition, should have been ignored, like old ACKs
(SEG.ACK < SND.UNA). The code still misses the management of the
mitigation of Blind Data Injection Attack.
This patch moves these switches inside ProcessEstablished, to simplify
the ReceivedAck function, which now manages only valid ACKS.
To complete the overview, one more condition has been added, which is
the case of an out-of-order ACK (SEG.ACK > SND.NXT).
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