Add thread safety documentation

src/server.cpp

@@ -391,7 +391,7 @@ void Server::start_accept_threads() {
                 perror("setsockopt SO_KEEPALIVE");
               }
 
-              auto conn = std::make_unique<Connection>(
+              auto conn = Connection::createForServer(
                   addr, fd,
                   connection_id_.fetch_add(1, std::memory_order_relaxed),
                   &handler_, weak_from_this());