Consumer.consume commits offsets when 'enable.auto.commit': False

Description
===========
When I disable auto commit with 'enable.auto.commit': False , and call poll(), the consumer doesn't commit and it's working fine, like it's said in the docs. But on consume() it commits offsets after every call. This is quite unexpected.
Consumer.close() is not called during execution.

How to reproduce
================

```
consumer = Consumer({'bootstrap.servers': 'server:9092',
'group.id': 'group_id',
'enable.auto.commit': False,
'auto.offset.reset': 'earliest'})
print(str(consumer.get_watermark_offsets(TopicPartition('topic_test', 0))))

try:
consumer.subscribe([config.KAFKA_INPUT_TOPIC])

while True:
messages = consumer.consume(num_messages=config.KAFKA_MAX_BATCH_SIZE, timeout=10.0)
if messages:
for msg in messages:
if msg.error():
if msg.error().code() == KafkaError._PARTITION_EOF:
# End of partition event
logger.error('%% %s [%d] reached end at offset %d\n' %
(msg.topic(), msg.partition(), msg.offset()))
else:
raise KafkaException(msg.error())
else:
print('message ready')

print(str(consumer.get_watermark_offsets(TopicPartition('topic_test', 0))))
#consumer.commit(asynchronous=False)
finally:
# Close down consumer to commit final offsets.
consumer.close()
```

Execution result: first call to get_watermark_offsets is different from all subsequent calls. I expect it to be the same. I need to get a batch of messages to process them concurrently, but seems like this batch consumpsion is committing the offsets before the processing which may result in loss of data.

Checklist
=========
Please provide the following information:

- [ ] confluent-kafka-python and librdkafka version (`confluent_kafka.version()` and `confluent_kafka.libversion()`): 1.8.2
- [ ] Apache Kafka broker version: 2.11
- [ ] Operating system: CentOS Linux release 7.9.2009