Compilation error with new Protocol Buffers version

Hi,

with the latest Protocol Buffer versions (starting from v3.12.0), the HotRod Client C++ build fails. (See the log in attach)

Looking at ProtoBuf release note (C++ section), it seems that from the aforementioned protobuf version, something is changed about this:

"Mark generated classes as final; inheriting from protos is strongly discouraged"