fix updating of Documents with event subscribers

fix for https://github.com/doctrine/phpcr-odm/issues/526

* [ ] add a test that converts a value back and forth via postLoad/prePersist events
* [ ] squash commits
i am pretty sure we now cover all the cases with the test. i also added another method to unschedule documents from updates. however this is just a performance optimization to a large extend, then again it should prevent needless events too which could prevent unwanted side effects.