Log warning about changes to main config if persisted changes have been done

If runtime changes have been done and a user modifies maxscale.cnf, the changes won't take effect after the next restart. This happens due to the persisted configuration files overwriting the user-defined values and is expected behavior. To inform the user of this fact, the existing warning that is logged should also explain that any changes to the main configuration file will not take effect and the changes should be done via maxctrl.

This could be done in a somewhat crude way by inspecting the modification timestamps of the files.