Garbage collection / Storage pruning

[shubertm]

When resource is deleted using other app or system, its properties still remain in storage. We need to clean storage after external resource deletion
ARK-Builders/Memo#8

[kirillt]

We had similar feature planned in Navigator :

• Garbage collection for tags storage Navigator#114

I'm not sure what is implementation status of it.. But we should not hurry with any garbage collection, because user-defined metadata (properties, tags, scores) is very important and we must not lose it. Better to have garbage in it but not remove something if index says it's not there any more. Another tricky spot is that resource could lag while syncing and properties can come earlier than the resource itself. In this case, we would erroneously remove the metadata.