SpkIterator was created with its own nth() and next() implementations
and its own new() and new_with_range() constructors.
Co-authored-by: 志宇 <hello@evanlinjin.me>
Introduce `chain_oracle::Cache` which is a cache for requests to the
chain oracle. `ChainOracle` has also been moved to the `chain_oracle`
module.
Introduce `get_tip_in_best_chain` method to the `ChainOracle` trait.
This allows for guaranteeing that chain state can be consistent across
operations with `IndexedTxGraph`.