Number of file handles does not scale with number of Consumer(s)

Description
===========
The number of open file handles is not proportional with the number of Consumer(s) in a process. I have a simple example where I create N `Consumer`(s) in a loop. For each N in [1...10] I run `lsof | awk '{print $4}' | sort | uniq -c | sort -n` to examine the number of open file handles. Below is a paste of the number of file handles for each N [1...10]

1: 531
2: 1386
3: 1848
4: 4068
5: 5896
6: 8046
7: 10518
8: 13320
9: 16443
10: 19890

Furthermore, why is it that > 500 file handles need to be opened for one Consumer?

How to reproduce
================
```
import time
import uuid

from confluent_kafka import Consumer, Producer, TopicPartition
from confluent_kafka.admin import AdminClient, NewTopic

if __name__ == "__main__":
conf = {"bootstrap.servers": "INSERT_ANY_BOOTSTRAP",
"enable.auto.commit": False,
"group.id": "kafka-test" + str(uuid.uuid1()),
}

tracker = []

for i in range(10):
topic = str(uuid.uuid1()) # can be any dummy topic name
consumer = Consumer(**conf)
consumer.subscribe([topic])
tracker.append(consumer)

while True:
time.sleep(5)
```
with
`lsof | awk '{print $4}' | sort | uniq -c | sort -n`

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

- [1.9.0 and 1.9.0 ] confluent-kafka-python and librdkafka version (`confluent_kafka.version()` and `confluent_kafka.libversion()`):
- [ ] Apache Kafka broker version:
- [ ] Client configuration: `{...}`
- [ ] Operating system:
- [ ] Provide client logs (with `'debug': '..'` as necessary)
- [ ] Provide broker log excerpts
- [ ] Critical issue