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`.

example-crates/wallet_electrum/src/main.rs

@@ -40,7 +40,7 @@ fn main() -> Result<(), anyhow::Error> {
 
     let prev_tip = wallet.latest_checkpoint();
     let keychain_spks = wallet
-        .spks_of_all_keychains()
+        .all_unbounded_spk_iters()
         .into_iter()
         .map(|(k, k_spks)| {
             let mut once = Some(());