Add a new configuration parameter to refresh the TTL of the session even if unchanged (#233)

Co-authored-by: Rob Ede <robjtede@icloud.com>

actix-session/tests/opaque_errors.rs

@@ -71,6 +71,10 @@ impl SessionStore for MockStore {
         todo!()
     }
 
+    async fn update_ttl(&self, _session_key: &SessionKey, _ttl: &Duration) -> Result<(), Error> {
+        todo!()
+    }
+
     async fn delete(&self, _session_key: &SessionKey) -> Result<(), Error> {
         todo!()
     }