Disk cache cleanup needs to disable when there's no upstream

[kf6kjg]

If there's no upstream asset server to forward assets to, then the server is implicitly in asset server mode - as opposed to asset cache mode. Thus it should be allowed to progress until it's taken over the disk array.

Maybe users should be warned? Maybe refuse to run if there's no upstream and limits have been set? That would imply being able to remove disk consumption limits...

EDIT: Why is disk cache cleanup a problem in this case? Because it means those assets will have been permanently deleted from the grid: no other server has them because there's no upstream server to forward them to.

[kf6kjg]

The goal has shifted: the code should be changed so the single-db LMDB edition is allowed to grow across multiple files like it's designed to do and block purging. The code should then be refactored to use the most relevant storage engine based on config.