Track loaded tree nodes with a special map.
No longer need to evict child nodes when evicting parent node. Before NodeMap, the only way to find a child node was through the parent. If any threads were accessing the child when the parent was evicted, the child could get loaded again, causing multiple instances of it to exist. See also: 90d8925b297129ac0611f74bb87b211688d03ff2
Version 1.1.5 notes.