fix internal path cache

[amitport]

Current internal path-cache implementation is error-prone (e.g., KeyPath.get("o.id") is not equal to KeyPath.get("o.['id']") and presents a memory leak (cache is never emptied).

Consider using something like memoize to avoid redundant path generation. Note that this means we cannot rely on instance to be intern and should implement an equals function for comparison (i.e., moving from keyPath === otherKeyPath to keyPath.equals(otherKeyPath).