Create a sub-folder for mySQL benchmarks

[lilith218]

Change

• move all mySQL benchmarks to bench/targets/mysql
• move all mySQL config files to config/mysql
• let cmake look recursively for headers and exclude those under sys
  use subdir name as target name prefix
• let bm-generator generate headers and config under subdir gen-ws
• reflect changes on related CI jobs and config files

[lilith218]

Create an issue to have a neater way to define or configure gen-ws. It should be controlled in one place.

[bvtrach]

I'm not sure I got all the implications of the change, but looks good to me so far. I'd let Martin approve too.

[martin-beck]

works like a charm