Ensure assets are downloaded just once

[petrsvihlik]

Avoids the "Multiple documents… output to" error.

Motivation

OCD (and also performance)

Checklist

• Code follows coding conventions held in this repo
• Automated tests have been added
• Tests are passing
• Docs have been updated (if applicable) N/A
• Temporary settings (e.g. variables used during development and testing) have been reverted to defaults

How to test

• create two or more pipelines with KontentImageProcessor whose output will contain assets () with the same URLs
• create another pipeline based on the KontentDownloadImages
• run dotnet run -- preview and make sure that:
  • there are no errors in the log such as Multiple documents output to img/1.jpg (this probably wasn't intended)
  • all assets are downloaded correctly

[petrsvihlik]

@alanta please let me know if the PR's up to your expectations :)

I might have a few more coming

[alanta]

Looks like a clean fix, thanks!