kvserver: remove/replace decommissioning and dead non-voters

[aayushshah15]

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.

[blathers-crl]

Hi @aayushshah15, please add a C-ategory label to your issue. Check out the label system docs.

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