cancel transaction on RLMRealm dealloc if still in write transaction

@tgoyne @alazier do you agree that this is a more appropriate behavior?
Something like "WARNING: A RLMRealm instance was deallocated while a write transaction was active, and the changes made have been rolled back. Make sure you are holding onto a reference to the RLMRealm for the duration of the write transaction." would be more helpful. Wording could use some work.