Are relations worth keeping?

Currently `OracleDocument` relation links are only utilised by the holoom block explorer. `Recipe`s do not support fetching documents by relation, because there is no way to prevent a an executor from censoring an item from the collection.

I can think of a somewhat convoluted approach to preventing censorship: A recipe names some future checkpoint document on an authority chain, and then once it exists, its action hash used as an upper bound for validating and aggregating relation information on the chain. However, it's worth questioning if this approach has significant advantages over the simpler approach of waiting for some future document list instead.

### Pros

- It removes an opportunity for an authority to retrospectively censor an item from the list they publish

### Cons

- It increases the complexity of aggregating documents sourced from multiple authorities, as future checkpoints would have to named per authority.