Define empty __slots__ in base classes

[anton-ryzhov]

I've noticed that __slots__ are defined in Message-classes, but they were not working as expected.

When inheriting from a class without slots, the dict and weakref attribute of the instances will always be accessible.

https://docs.python.org/3/reference/datamodel.html?#object.__slots__

So all parent classes must also have __slots__ to disable __dict__ creation. That's what I did in this PR

[coveralls]

coverage: 88.434% (+0.03%) from 88.402%
when pulling c4d59b1 on anton-ryzhov:define-slots
into 3352ce4 on mosquito:master.

[decaz]

@anton-ryzhov can you add __slots__ for parent classes of RobustQueue which also has this attribute, please?

[anton-ryzhov]

Done