fix: improve more docs and more refactoring

Thank you @vladimirfomene for these suggestions.

* Rename `LocalUpdate::keychain` to `LocalUpdate::last_active_indices`.
* Change docs for `CheckPoint` to be more descriptive.
* Fix incorrect logic in `update_local_chain` for `EsploraExt` and
  `EsploraAsyncExt`.

crates/esplora/src/async_ext.rs

@@ -282,7 +282,6 @@ impl EsploraAsyncExt for esplora_client::AsyncClient {
                     async move { client.get_tx_status(&txid).await.map(|s| (txid, s)) }
                 })
                 .collect::<FuturesOrdered<_>>();
-            // .collect::<Vec<JoinHandle<Result<(Txid, TxStatus), Error>>>>();
 
             if handles.is_empty() {
                 break;