[KafkaIO] Determine partition backlog using endOffsets instead of seek2End and position

[sjvanrossum]

Use endOffsets instead of seekToEnd and position to determine the backlog element size in KafkaUnboundedReader and ReadFromKafkaDoFn. Determining the end offsets by seeking and retrieving the position may trigger poll and prefetch records which will never be consumed by the offset consumer.

Forcibly retrieving the partition's high watermark in offset consumers might not be necessary, but this will be addressed separately.

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[Abacn]

Hi, have you been able to get a KafkaIO reviewer? Reviewbot possibly needs a Kafka label.