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

Also fixes ISPN-2150 "Data Rehash Event: getMembersAtStart/End returns empty set"

* 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).

* Remove membersAtStart, membersAtEnd collections from EventImpl. These can be derived from the CHes 

* Re-implement EventImpl.getMembersAtStart/End to return the members from the corresponding CH if not null or return an empty collection otherwise 

JIRA: https://issues.jboss.org/browse/ISPN-2764 ,
         https://issues.jboss.org/browse/ISPN-2150