@Override
        public String toString() {
            return replicaId + " (" + reason + ")";
        }
Updates the directory to partition mapping for a single partition.
Assignments to reserved directory IDs are ignored, since they cannot
be used for directories, there's no use in maintaining a set of
partitions assigned to them.
metadata/src/main/java/org/apache/kafka/controller/ReplicationControlManager.java