Using an empty struct instead of void* to represent the opaque
filter and filter session types, means that C++ filter types
simply can derive from them. That brings conceptual clarity and
also means that the C++ instances can be returned from the module
callbacks without casts. When provided as argument they can be
downcast using static_cast instead of reinterpret_cast.
In a subsequent change, the prototypes of MXS_ROUTER_OBJECT will
be updated and routers modified accordingly.
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