Failed shards could be re-assigned to the same nodes if multiple replicas failed at once

After a shard fails on a node we assign a new replica on another node. This is important in order to avoid failing again due to node specific problems. In the rare case where two different replicas of the same shard failed in a short time span, we may fail to do so and assign one of them back to the node it's currently on. This happens if both shard failed events are processed within the same batch on the master.