future direction of the mysql CNID backend

[rdmark]

these days it's arguably more likely that you use the MariaDB client library with our mysql backend than the MySQL client library, because the former is FLOSS and more broadly distributed in open source operating systems; it's the former that I use consistently for testing

also, the two libraries are gradually being developed in different directions; are there emerging unique features in either that would provide a performance boost?

two potential paths:

• keep a universal backend, and potentially rename it to a neutral name
• split into two separate backends, optimized for each database engine