dccp ccid-2: Algorithm to update buffer state

This provides a routine to consistently update the buffer state when the
peer acknowledges receipt of Ack Vectors; updating state in the list of Ack
Vectors as well as in the circular buffer.

While based on RFC 4340, several additional (and necessary) precautions were
added to protect the consistency of the buffer state. These additions are
essential, since analysis and experience showed that the basic algorithm was
insufficient for this task (which lead to problems that were hard to debug).

The algorithm now
 * deals with HC-sender acknowledging to HC-receiver and vice versa,
 * keeps track of the last unacknowledged but received seqno in tail_ackno,
 * has special cases to reset the overflow condition when appropriate,
 * is protected against receiving older information (would mess up buffer state).

Note: The older code performed an unnecessary step, where the sender cleared
Ack Vector state by parsing the Ack Vector received by the HC-receiver. Doing
this was entirely redundant, since
 * the receiver always puts the full acknowledgment window (groups 2,3 in 11.4.2)
   into the Ack Vectors it sends; hence the HC-receiver is only interested in the
   highest state that the HC-sender received;
 * this means that the acknowledgment number on the (Data)Ack from the HC-sender
   is sufficient; and work done in parsing earlier state is not necessary, since
   the later state subsumes the  earlier one (see also RFC 4340, A.4).
This older interface (dccp_ackvec_parse()) is therefore removed.

Signed-off-by: Gerrit Renker <gerrit@erg.abdn.ac.uk>

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