Add on_write_progress

src/connection.hpp

@@ -44,6 +44,13 @@ struct Connection {
   void closeAfterSend() { closeConnection_ = true; }
   ArenaAllocator &getArena() { return arena_; }
   int64_t getId() const { return id_; }
+  int64_t outgoingBytesQueued() const {
+    int64_t result = 0;
+    for (auto s : messages_) {
+      result += s.size();
+    }
+    return result;
+  }
 
   // Note: To release connection back to server, use
   // Server::releaseBackToServer(std::move(connection_ptr))