Inspired by https://github.com/blue-systems/plasma-5.16/issues/139#issuecomment-441704579
After some review, it seems to me using KConfig is not necessary here.
Removing its use should improve IO throughput since rebuildConfig is called in finished().
So mark the old config() function as deprecated.
- port ioslaves in kio and kio-extras to use mapConfig() instead of config()
- Clean up this description