refactor(chain)!: revamp `KeychainTxOutIndex` API

refactor(chain)!: revamp `KeychainTxOutIndex` API

Previously `SpkTxOutIndex` methods can be called from
`KeychainTxOutIndex` due to the `DeRef` implementation. However, the
internal `SpkTxOut` will also contain lookahead spks resulting in an
error-prone API.

`SpkTxOutIndex` methods are now not directly callable from
`KeychainTxOutIndex`. Methods of `KeychainTxOutIndex` are renamed for
clarity. I.e. methods that return an unbounded spk iter are prefixed
with `unbounded`.