Prefer Arc::clone(&a) to a.clone()

It enhances the readability since it shows clone function creates a new
reference and does not copy the entire object.

https://doc.rust-lang.org/std/sync/struct.Arc.html#cloning-references

Author: Seulgi Kim

state/src/db/state_db.rs

@@ -369,15 +369,15 @@ impl StateDB {
     pub fn clone_canon(&self, parent: &H256) -> StateDB {
         StateDB {
             db: self.db.boxed_clone(),
-            account_cache: self.account_cache.clone(),
-            regular_account_cache: self.regular_account_cache.clone(),
-            metadata_cache: self.metadata_cache.clone(),
-            shard_cache: self.shard_cache.clone(),
-            shard_metadata_cache: self.shard_metadata_cache.clone(),
-            world_cache: self.world_cache.clone(),
-            asset_scheme_cache: self.asset_scheme_cache.clone(),
-            asset_cache: self.asset_cache.clone(),
-            action_data_cache: self.action_data_cache.clone(),
+            account_cache: Arc::clone(&self.account_cache),
+            regular_account_cache: Arc::clone(&self.regular_account_cache),
+            metadata_cache: Arc::clone(&self.metadata_cache),
+            shard_cache: Arc::clone(&self.shard_cache),
+            shard_metadata_cache: Arc::clone(&self.shard_metadata_cache),
+            world_cache: Arc::clone(&self.world_cache),
+            asset_scheme_cache: Arc::clone(&self.asset_scheme_cache),
+            asset_cache: Arc::clone(&self.asset_cache),
+            action_data_cache: Arc::clone(&self.action_data_cache),
 
             local_account_cache: LocalCache::new(),
             local_regular_account_cache: LocalCache::new(),
@@ -464,15 +464,15 @@ impl Clone for StateDB {
     fn clone(&self) -> Self {
         Self {
             db: self.db.boxed_clone(),
-            account_cache: self.account_cache.clone(),
-            regular_account_cache: self.regular_account_cache.clone(),
-            metadata_cache: self.metadata_cache.clone(),
-            shard_cache: self.shard_cache.clone(),
-            shard_metadata_cache: self.shard_metadata_cache.clone(),
-            world_cache: self.world_cache.clone(),
-            asset_scheme_cache: self.asset_scheme_cache.clone(),
-            asset_cache: self.asset_cache.clone(),
-            action_data_cache: self.action_data_cache.clone(),
+            account_cache: Arc::clone(&self.account_cache),
+            regular_account_cache: Arc::clone(&self.regular_account_cache),
+            metadata_cache: Arc::clone(&self.metadata_cache),
+            shard_cache: Arc::clone(&self.shard_cache),
+            shard_metadata_cache: Arc::clone(&self.shard_metadata_cache),
+            world_cache: Arc::clone(&self.world_cache),
+            asset_scheme_cache: Arc::clone(&self.asset_scheme_cache),
+            asset_cache: Arc::clone(&self.asset_cache),
+            action_data_cache: Arc::clone(&self.action_data_cache),
 
             local_account_cache: LocalCache::new(),
             local_regular_account_cache: LocalCache::new(),