fix(chain): avoid using BTreeMap::append

The implementation of `BTreeMap::append` is non-performant making
merging changesets very slow. We use `Extend::extend` instead.

Refer to:
rust-lang/rust#34666 (comment)

Author: evanlinjin

crates/chain/src/keychain.rs

@@ -58,8 +58,9 @@ impl<K: Ord> Append for ChangeSet<K> {
                 *index = other_index.max(*index);
             }
         });
-
-        self.0.append(&mut other.0);
+        // We use `extend` instead of `BTreeMap::append` due to performance issues with `append`.
+        // Refer to https://github.com/rust-lang/rust/issues/34666#issuecomment-675658420
+        self.0.extend(other.0);
     }
 
     /// Returns whether the changeset are empty.

crates/chain/src/tx_data_traits.rs

@@ -123,8 +123,10 @@ pub trait Append {
 }
 
 impl<K: Ord, V> Append for BTreeMap<K, V> {
-    fn append(&mut self, mut other: Self) {
-        BTreeMap::append(self, &mut other)
+    fn append(&mut self, other: Self) {
+        // We use `extend` instead of `BTreeMap::append` due to performance issues with `append`.
+        // Refer to https://github.com/rust-lang/rust/issues/34666#issuecomment-675658420
+        BTreeMap::extend(self, other)
     }
 
     fn is_empty(&self) -> bool {
@@ -133,8 +135,10 @@ impl<K: Ord, V> Append for BTreeMap<K, V> {
 }
 
 impl<T: Ord> Append for BTreeSet<T> {
-    fn append(&mut self, mut other: Self) {
-        BTreeSet::append(self, &mut other)
+    fn append(&mut self, other: Self) {
+        // We use `extend` instead of `BTreeMap::append` due to performance issues with `append`.
+        // Refer to https://github.com/rust-lang/rust/issues/34666#issuecomment-675658420
+        BTreeSet::extend(self, other)
     }
 
     fn is_empty(&self) -> bool {

crates/chain/src/tx_graph.rs

@@ -1271,10 +1271,12 @@ impl<A> ChangeSet<A> {
 }
 
 impl<A: Ord> Append for ChangeSet<A> {
-    fn append(&mut self, mut other: Self) {
-        self.txs.append(&mut other.txs);
-        self.txouts.append(&mut other.txouts);
-        self.anchors.append(&mut other.anchors);
+    fn append(&mut self, other: Self) {
+        // We use `extend` instead of `BTreeMap::append` due to performance issues with `append`.
+        // Refer to https://github.com/rust-lang/rust/issues/34666#issuecomment-675658420
+        self.txs.extend(other.txs);
+        self.txouts.extend(other.txouts);
+        self.anchors.extend(other.anchors);
 
         // last_seen timestamps should only increase
         self.last_seen.extend(