[$] DNF 3: better performance and a move to C++

It has only been a few years since DNF replaced Yum as the default Fedorapackage-management tool; that was done for Fedora 22 in 2015, thoughDNF had been available for several earlier Fedora releases. Since thattime, DNF development has proceeded; it started a move from Python/C to all C in2016 and has made multiple releases over the years. From an outsider'sperspective, no major changes seem necessary, which makes the announcementof DNF 3, and a move to C++, a bit surprising to some.