Added a function to determine if bootstrap is successfully connected

[Wayde2014]

Added a function to determine if bootstrap is successfully connected

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This change is 

[dpkp]

I like the idea! Rather than adding more state to the KafkaProducer (self._client), maybe delegate this function to the Sender class, which owns the client instance?

[Wayde2014]

@dpkp I tested your optimized bootstrap_connected function, then found that it was not what I wanted.
However, when I changed the bootstrap_connected function as the following code, it work correctly.

    def bootstrap_connected(self):
        """Return True if a bootstrap node is connected"""
        if self._bootstrap_fails > 0:
            return False
        return True

old code:

    def bootstrap_connected(self):
        """Return True if a bootstrap node is connected"""
        for node_id in self._conns:
            if not self.cluster.is_bootstrap(node_id):
                continue
            if self._conns[node_id].connected():
                return True
        else:
            return False