* Remove `chain_oracle::CacheBackend` for now as it is not used. * `SparseChain` does not need to implement `ChainOracle`. * Remove filter predicate for `list..` methods of `TxGraph` and `IndexedTxGraph` as this is premature optimisation. * `Append` can be implemented for all `BTreeMap`s and `BTreeSet`s, instead of only `local_chain::ChangeSet`.