Cleanup the active_writes set too.

celery/concurrency/asynpool.py

@@ -571,11 +571,13 @@ class AsynPool(_pool.Pool):
                 busy_workers.discard(inq)
             hub_remove(proc.sentinel)
             waiting_to_start.discard(proc)
+            self._active_writes.discard(proc.inqW_fd)
             hub_remove(proc.inqW_fd)
             hub_remove(proc.outqR_fd)
             if proc.synqR_fd:
                 hub_remove(proc.synqR_fd)
             if proc.synqW_fd:
+                self._active_writes.discard(proc.synqW_fd)
                 hub_remove(proc.synqW_fd)
         self.on_process_down = on_process_down