Fix inefficient use of __slots__ in various classes

While investigating PYTHON-1317 I realized that our _WriteOp subclasses do not define __slots__. This means these classes will automatically include __dict__. From the Python docs:

The action of a _slots_ declaration is limited to the class where it is defined. As a result, subclasses will have a _dict_ unless they also define _slots_ (which must only contain names of any additional slots).

https://docs.python.org/2/reference/datamodel.html#__slots__

>>> from pymongo.operations import InsertOne

>>> i = InsertOne({})

>>> i.__dict__

{}

This same problem is present in a number of other class hierarchies, including _WriteResult, _ServerMode, _ServerHeartbeatEvent, and TopologyEvent. We should fix up our usage of slots so that it does what we intend.