StateTransferManagerImpl.waitForInitialStateTransferToComplete() returns too soon

StateTransferManagerImpl.waitForInitialStateTransferToComplete() returns as soon as a joining node confirmed to the coordinator that it received all the data it needed (see STMI.notifyEndOfTopologyUpdate()).

It should return only after the coordinator has confirmed the end of the rebalance with a new topology update (see STMI.doTopologyUpdate()).

This should make it more likely for the tests suite clusters to be in a stable state by the time the test starts, and should help with the random state transfer-related failures in non-state transfer tests.

Instead we should make sure that we do have tests that check forwarding behaviour explicitly.