We need to address this TODO:
https://github.com/cockroachdb/cockroach/blob/master/pkg/kv/kvserver/allocator.go#L561-L562

Currently, the allocator will not spit out the right actions to remove / replace decommissioning or dead non-voter replicas. This means that, for instance, a decommissioning process on a node can get stuck when the node has non-voters on it.