PreferAvailabilityStrategy conflict resolution should occur if an expected member is not in max topology

The current method for determining whether a split brain heal is occurring during an on merge event is wrong. Instead we should assume healing whenever one or more of the expected members is not in the membership of the maxTopology received.

Furthermore, we should also utilise the Union CH of all distinct CHs encountered for conflict resolution. This is necessary to ensure that we read the entries associated with all possible write owners before the rebalance occurs.