[Bugfix][Frontend] Fix Issues Under High Load With zeromq Frontend (#7394)

Co-authored-by: Nick Hill <nickhill@us.ibm.com>

vllm/engine/async_llm_engine.py

@@ -766,6 +766,11 @@ class AsyncLLMEngine:
     def errored(self) -> bool:
         return self._errored_with is not None
 
+    @property
+    def limit_concurrency(self) -> Optional[int]:
+        """Maximum number of concurrently running requests."""
+        return None
+
     def set_errored(self, exc: Exception) -> None:
         self._errored_with = exc