Silence exceptions when cluster is shutting down.

I'm not sure if this is the best patch, but I did notice that close() was called elsewhere so I figured just to check for variable existence. Worst case: it showcases where the exceptions on cluster.shutdown() were coming from.

Here are the exceptions that I was getting on a normal program exit:

    Exception AttributeError: AttributeError("'NoneType' object has no attribute 'debug'",) in <bound method Session.__del__ of <cassandra.cluster.Session object at 0x10e50bc10>> ignored
    Exception AttributeError: "'NoneType' object has no attribute 'debug'" in <bound method Cluster.__del__ of <cassandra.cluster.Cluster object at 0x10e4c8ad0>> ignored

I realized that log was None, much like:
https://github.com/datastax/python-driver/blob/master/cassandra/cluster.py#L1526

As were \_loop\_lock and ConnectionShutdown.