Use DataRehashedEvent to signal end of state transfer

Since NBST was introduced the actual completion of state transfer happens later than the TopologyChangedEvent. In unit tests there is currently no simple way to find out when this actually happens so we resorted to some ugly polling (see TestingUtil.waitForRehashToComplete()). Also some other core components could benefit from such an event (eg. StaleTransactionCleanupService, see ISPN-2383).

The event should contain the CacheTopology that is being confirmed as stable.