DNF 3: better performance and a move to C++

I'm not sure about the graceful failure handling.
Sometimes it's better to report an error than ignore it and exit with 0.
That could make a false assumption that everything works perfectly.

About the plugins support - we do *not* have any plans to remove them from Python DNF.
On the other hand, it would be great to support plugins directly in libdnf,
in order to make them available to all tools based on the library.
We can't link libdnf with Python to keep the footprint and deps small.
Once the software management stack consolidation is finished,
we'll definitely look into this. Now it's early to say how the solution will look like.