refactor s3 provider

src/comm/packet/client_pool.cpp

@@ -148,8 +148,8 @@ void client_pool::shutdown() {
     unique_mutex_lock pool_lock(pool_mutex_);
     if (not shutdown_) {
       shutdown_ = true;
-      for (auto &pool : pool_lookup_) {
-        pool.second->shutdown();
+      for (auto &pool_entry : pool_lookup_) {
+        pool_entry.second->shutdown();
       }
       pool_lookup_.clear();
     }