message: Fix warning "explicitly defaulted move assignment operator is implicitly deleted "
  Objects with a reference member cannot be assigned to.
diff: @@ -34,7 +34,6 @@ public:
  static MemRef create_empty_cluster(Allocator& alloc);
  ClusterTree(ClusterTree&&) = default;
  - ClusterTree& operator=(ClusterTree&&) = default;
  // Disable copying, this is not allowed.
  ClusterTree& operator=(const ClusterTree&) = delete;
  