1. Implement a forward chain iterator that should provide a snapshot view of elder membership, having API like this:
2. Consider the implementation of backwards iteration Chain Iterator
3. Figure out the return type of this function (should it be PeerId, PublicId, or just a PublicKey?). If it's PublicId then consider that there's an age field that might affect the result.
4. Depends on the node startup process, the forward and backward iterators could be both required or one of them.
5. Implement PartialEq trait for DataChain. Two chains are considered equal if they contain the same blocks, albeit not in the same order.