Optimize database usage

[aprokopenko]

All database tables is based on "image size" of an attachment. But several image sizes can have same file source. This issue create duplicates in stats. (This issue were found on big sites with lot of image sizes after the plugin is ready)

For now to fix this we used more complex sql queries to skip duplicates. More logical is to re-write saving logic to be based on file sources, not image sizes.