storage: add background task to periodically clean up data in S3

[sanderegg]

Is there an existing issue for this?

• I have searched the existing issues

Planned changes

Since we do now have folders inside S3, the synchronisation between postgres and S3 is most probably broken in at least the following use-cases:

• deleting folders and storage is restarted, or dead-locked, etc... --> S3 files are lying there forever which implies costs

Therefore it is desirable to have a background task that runs with low priority and checks that the S3 contents matches the database, for instance:

• any /node_id that does not exist in the db should be removed
• any /project_id that does not exist in the db should be removed
• also be careful of current uploads

This is a sub issue from ITISFoundation/osparc-issues#675