feat(chain): KeychainTxOutIndex: Make spk cache optional

Also added staging changes to `ChangeSet::spk_cache`. This way, we can
avoid returning `ChangeSet`s for `apply_changeset` and
`insert_descriptor`.

* `KeychainTxOutIndex::new` now takes in an additional `use_spk_cache`
  parameter.
* Fixed `reveal_to_target` method to actually return `None` if the
  keychain does not exist.

Author: evanlinjin

crates/bitcoind_rpc/examples/filter_iter.rs

@@ -33,9 +33,8 @@ fn main() -> anyhow::Result<()> {
     let (mut chain, _) = LocalChain::from_genesis_hash(genesis_block(NETWORK).block_hash());
     let mut graph = IndexedTxGraph::<ConfirmationBlockTime, KeychainTxOutIndex<&str>>::new({
         let mut index = KeychainTxOutIndex::default();
-        // TODO: Properly deal with these changesets.
-        let _ = index.insert_descriptor("external", descriptor.clone())?;
-        let _ = index.insert_descriptor("internal", change_descriptor.clone())?;
+        index.insert_descriptor("external", descriptor.clone())?;
+        index.insert_descriptor("internal", change_descriptor.clone())?;
         index
     });
 

crates/chain/benches/canonicalization.rs

@@ -82,8 +82,8 @@ fn setup<F: Fn(&mut KeychainTxGraph, &LocalChain)>(f: F) -> (KeychainTxGraph, Lo
 
     let (desc, _) =
         <Descriptor<DescriptorPublicKey>>::parse_descriptor(&Secp256k1::new(), DESC).unwrap();
-    let mut index = KeychainTxOutIndex::new(10);
-    let _ = index.insert_descriptor((), desc).unwrap();
+    let mut index = KeychainTxOutIndex::new(10, true);
+    index.insert_descriptor((), desc).unwrap();
     let mut tx_graph = KeychainTxGraph::new(index);
 
     f(&mut tx_graph, &chain);