EC: ReplicationManager - Track nodes already used when handing under replication

If a container has a missing replica, and some decommission replicas, then it will have both a reconstruction and replication command scheduled when it is processed for under replication.

However, the target node picked for the reconstruction is not excluded when picking nodes for the replication, so it would be possible for both commands to target the same node with different indexes.

We need to track all nodes selected when processing under replication to ensure they are not used twice.