Release autoreleasepool in completion queue loop

src/objective-c/GRPCClient/private/GRPCCore/GRPCCompletionQueue.m

@@ -56,6 +56,7 @@ const grpc_completion_queue_attributes kCompletionQueueAttr = {
     });
     dispatch_async(gDefaultConcurrentQueue, ^{
       while (YES) {
+        @autoreleasepool {
           // The following call blocks until an event is available.
           grpc_event event =
               grpc_completion_queue_next(unmanagedQueue, gpr_inf_future(GPR_CLOCK_REALTIME), NULL);
@@ -71,6 +72,7 @@ const grpc_completion_queue_attributes kCompletionQueueAttr = {
             default:
               [NSException raise:@"Unrecognized completion type" format:@""];
           }
+        }
       };
     });
   }