PruneCallback is a function which can be provided by the user which is called for each CID when the last height at which that CID appears is pruned.

TrackBlobsFun is passed to the UpdateFn provided to Storage.Update, and can be called to track a list of cids at a given block height.

UpdateFn is implemented by the user and passed to Storage.Update, which ensures that it will never be run concurrently with any call to Storage.Prune.