New version of patch:

1. Use RLock instead of Lock (Mark & Antoine recomendation)
2. Add lock acquire at `__setitem__` and `__delitem__` as following (Mark & Antoine recomendation)
3. Add lock protection in `__repr__` implementation.

I've some questions pending:

1. Is it a good idea to wrap self._data access with lock in __getitem__? (I suspect it's not)
2. Is it a good idea to lock protect self.copy() and self.setdefault() (I suspect it's not because both uses __iter__ that is already protected)