ShardingTest::awaitBalancerRound() doesn't work in case of CSRS stepdowns

awaitBalancerRound works by polling on `balancerStatus` command and see when the number of rounds increase by one.

If a new primary of the configsvr is elected this approach is wrong, in fact  each replicaset node of the CSRS is keeping its count of balancer rounds, so this means that there is no guarantee that the number of rounds returned by `balancerStatus` is an always increasing number.