feat: make MethodWeights cheap clone (AcalaNetwork#16)

src/extensions/rate_limit/weight.rs

@@ -1,25 +1,23 @@
 use crate::config::RpcMethod;
 use std::collections::BTreeMap;
+use std::sync::Arc;
 
 #[derive(Clone, Debug, Default)]
-pub struct MethodWeights(BTreeMap<String, u32>);
+pub struct MethodWeights(Arc<BTreeMap<String, u32>>);
 
 impl MethodWeights {
-    pub fn add(&mut self, method: &str, weight: u32) {
-        self.0.insert(method.to_owned(), weight);
-    }
-
     pub fn get(&self, method: &str) -> u32 {
         self.0.get(method).cloned().unwrap_or(1)
     }
 }
 
 impl MethodWeights {
     pub fn from_config(methods: &[RpcMethod]) -> Self {
-        let mut weights = MethodWeights::default();
+        let mut weights = BTreeMap::default();
         for method in methods {
-            weights.add(&method.method, method.rate_limit_weight);
+            weights.insert(method.method.to_owned(), method.rate_limit_weight);
         }
-        weights
+
+        Self(Arc::new(weights))
     }
 }