Cluster Manager: further improvements to "fix": - clusterManagerFixOpenSlot: ensure that the   slot is unassigned before ADDSLOTS - clusterManagerFixSlotsCoverage: after cold   migration, the slot configuration   is now updated on all the nodes.
Cluster Manager: further improvements to "fix":
- clusterManagerFixOpenSlot: ensure that the
  slot is unassigned before ADDSLOTS
- clusterManagerFixSlotsCoverage: after cold
  migration, the slot configuration
  is now updated on all the nodes.

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