Large CPU usage due to busy waiting by KafkaConsumer.poll()

[braedon]

KafkaConsumer.poll() is effectively busy waiting, as KafkaConsumer._poll_once() doesn't respect timeout_ms.

_poll_once() sets timeout_ms of KafkaClient.poll() to:

min(timeout_ms, self._coordinator.time_to_next_poll())

However, ConsumerCoordinator.time_to_next_poll() returns the time in seconds, not milliseconds. The effective timeout is always much lower than timeout_ms, causing KafkaConsumer.poll() to spin, consuming a lot of CPU.