ISPN-2764 DataRehashEvent with pre == false has null getConsistentHashAtEnd()

StateConsumerImpl should record the CH at the time rebalance was started and the current CH when it ends and use these for broadcasting the DataRehashedEvent(pre==false). The old code was broadcasting currentCH (for oldCH) and null (for newCH).
ISPN-2150 Data Rehash Event: getMembersAtStart/End returns empty set

* Remove membersAtStart, membersAtEnd collections from EventImpl. These can be derived from the CHes
* Re-implement getMembersAtStart/End to return the members from the corresponding CH if not null or return an empty collection otherwise
* Remove unused methods from EventImpl