Close event channel when consumer is closed

[czchen]

This PR let Consumer to close consumer.events channel when calling Consumer.Close(). In this way, we can use the following code to consume data from Consumer, and exit automatically when Consumer.Close() is called.

for ev := range consumer.Events() {
    // do something here
}

[ghost]

@confluentinc It looks like @czchen just signed our Contributor License Agreement. 👍

Always at your service,

clabot

[edenhill]

Good fix, thanks!