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/chain/tests/test_local_chain.rs

@@ -123,7 +123,7 @@ fn update_local_chain() {
             },
         },
         // Introduce an older checkpoint (A) that is not directly behind PoA
-        //        | 1 | 2 | 3
+        //        | 2 | 3 | 4
         // chain  |     B   C
         // update | A       C
         TestLocalChain {