ISPN-4679 need to check if bundle is null from weak reference
Change ReplicatedCHFactory to only replace leavers in updateMembers()

Uneven primary ownership should be dealt with in rebalance() only.
Delegate the isBalanced() check in DefaultRebalancePolicy to the CH factory

Allows waitForRehashToComplete() to work even if the CH factory allocated
less than numOwners owners to each segment.