cancel transaction on RLMRealm dealloc if still in write transaction

@tgoyne @alazier do you agree that this is a more appropriate behavior?
I like the wrapping, but this'll result in a bunch of spaces in the log message. This can be fixed with:
```
NSLog(@"WARNING: An RLMRealm instance was deallocated during a write transaction and all "
       "pending changes have been rolled back. Make sure to retain a reference to the "
       "RLMRealm for the duration of the write transaction.");
```