Consumer.get_watermark_offsets Ignores Timeout Parameter in Some Situations

Description
===========
The get_watermark_offsets ignores the timeout and blocks forever when the kafka broker for a selected partition is down. Once the broker is back up, the function returns. I suspect this is a bug in rdkafka, but I'm posting it here first in case it's an issue with the python bindings.

How to reproduce
================
Create a topic with one partition. Run the below example. Once the example has consumed a few messages, kill the broker hosting the partition. The call to get_watermark_offsets will block until the broker comes back up.

```python
import time
import sys
import confluent_kafka
from confluent_kafka import Consumer, KafkaError
from uuid import uuid4

if __name__ == '__main__':
debug_thread = threading.Thread(target=debug_thread_func)
debug_thread.start()

client = Consumer({'bootstrap.servers': 'gateway:9092', 'group.id': str(uuid4()),
'default.topic.config': {'auto.offset.reset': 'smallest'}})

def assigned(consumer, partitions):
print("Assigned:", partitions)

client.subscribe(['ibbot'], on_assign=assigned)

while True:
msg = client.poll(timeout=1)
for partition in client.assignment():
print(client.get_watermark_offsets(partition, timeout=1))

if msg is not None:
if msg.error():
print("Error: ", msg.error())
else:
print("Data")

client.close()

```

Note that this example is probably dependent on a bug in rdkafka, so it may not be reproducible 100% of the time. You can also reproduce this with a multi-partition and broker setup. In this case, the function only blocks for as long as it takes for a new leader to be elected.

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

- [x] confluent-kafka-python and librdkafka version (`confluent_kafka.version()` and `confluent_kafka.libversion()`): kafka-python: ('0.11.4', 721920) rdkafka: ('0.11.4', 722175)
- [x] Apache Kafka broker version: 0.11.0.2
- [x] Client configuration: `{'bootstrap.servers': 'gateway:9092', 'group.id': str(uuid4()),
'default.topic.config': {'auto.offset.reset': 'smallest'}}`
- [X] Operating system: RHEL 7
- [ ] Provide client logs (with `'debug': '..'` as necessary)
- [ ] Provide broker log excerpts
- [ ] Critical issue