Better handling of `close()` when called from `__del__`
Better handling of `close()` when called from `__del__`

```
Exception AttributeError: "'AsyncReqMessageClient' object has no attribute '_closing'" in <bound method AsyncReqMessageClient.__del__ of <salt.transport.zeromq.AsyncReqMessageClient object at 0x1a0e82e90>> ignored
```

saltstack/salt,saltstack/salt,saltstack/salt,saltstack/salt,saltstack/salt