Adds a store_mut method to kademlia. (#1192)

* Adds a retain method to kademlia.

* Add a store_mut getter to Kademlia

* Removes a blank line

* Changes store_mut comment appropriately

* Fixes build

* Return a type, not a trait

protocols/kad/src/behaviour.rs

@@ -276,6 +276,11 @@ where
         }
     }
 
+    /// Get a mutable reference to internal record store.
+    pub fn store_mut(&mut self) -> &mut TStore {
+        &mut self.records
+    }
+
     /// Bootstraps the local node to join the DHT.
     ///
     /// Bootstrapping is a multi-step operation that starts with a lookup of the local node's