Deletion/Gc

I'd like to be able to use a blob store like this to store things with the option of purging data that should no longer be kept (e.g. for privacy reasons). But right now the interfaces in this package don't provide any way to delete blobs.

So, I have some interest in that feature, but additionally it would be nice if the store could keep track of edges between blobs and do garbage collection, treating the anchors as roots.

Start by reading the package interfaces that currently expose blob storage, then trace how blobs and anchors are represented. Define how explicit deletion, retention for privacy, blob edges, and garbage collection from anchors should work; done requires an agreed interface and behavior for purging unreachable blobs.

Written by the indexing model from the issue text.