[entity] allow null entityPreloadIterator (BC break!)
[mapper] oneHasMany: fixed iterating over entities with empty entityPreloadIterator
[tests] added basic manyHasOne test
[mapper] manyHasOne: fixed iterating over entities with empty entityPreloadIterator
[mapper] manyHasMany: fixed iterating over entities with empty entityPreloadIterator
[mapper] caching keys are md5ed - more memory efficient