ref(chain): Refactor next_store_index

Use `keys() + filter(...)` instead of `range(...)` and rename `v` to
`index` for better clarity.

Author: danielabrozzoni

crates/chain/src/keychain/txout_index.rs

@@ -206,9 +206,10 @@ impl<K: Clone + Ord + Debug> KeychainTxOutIndex<K> {
     fn next_store_index(&self, keychain: &K) -> u32 {
         self.inner()
             .all_spks()
-            .range((keychain.clone(), u32::MIN)..(keychain.clone(), u32::MAX))
+            .keys()
+            .filter(|(k, _)| k == keychain)
             .last()
-            .map_or(0, |((_, v), _)| *v + 1)
+            .map_or(0, |(_, index)| *index + 1)
     }
 
     /// Generates script pubkey iterators for every `keychain`. The iterators iterate over all