GMS: view acks on MergeView are only required from partition coordinators

When we have a merge of

and

, and A and X are the partition coordinators, then view acks for the new combined view

will not be received from non coordinator B,C, Y and Z.
Therefore, as an optimzation, and to avoid spurious warnings, we should get acks for the MergeView only from A and X. A itself will dissemniate the view locally and should only get acks from A,B,C, and X should only get acks from X,Y,Z, too.