producer/kafka: Disable logging during object destruction

Logging in an object destructor leads to crashes like the following

```
Exception ignored in: <bound method KafkaProducer.__del__ of <kafka.producer.kafka.KafkaProducer object at 0x7f0c47309b38>>
Traceback (most recent call last):
  File "/usr/local/lib/python3.6/site-packages/kafka/producer/kafka.py", line 447, in __del__
  File "/usr/local/lib/python3.6/site-packages/kafka/producer/kafka.py", line 460, in close
  File "/usr/local/lib/python3.6/logging/__init__.py", line 1305, in info
  File "/usr/local/lib/python3.6/logging/__init__.py", line 1546, in isEnabledFor
TypeError: '>=' not supported between instances of 'int' and 'NoneType'
```

In issue dpkp#1218 and dpkp#1445, @dpkp made clear that producers should be
closed during while they are destructed.

This commit does not modify the existing architecture and code structure,
and preserves all the logging calls in `close`. At the same time, by
"nullifying" the logger, it fixes the crash that is inherent to the use
of logging methods during the destruction of an object.

Author: gioele

kafka/producer/kafka.py

@@ -445,6 +445,14 @@ def _unregister_cleanup(self):
         self._cleanup = None
 
     def __del__(self):
+        # Disable logger during destruction to avoid touching dangling references
+        class NullLogger(object):
+            def __getattr__(self, name):
+                return lambda *args: None
+
+        global log
+        log = NullLogger()
+
         self.close()
 
     def close(self, timeout=None):