Hang if incremental assign is received while consumer is closing

[emasab]

Description

Consumer in cooperative incremental mode will hang if an assignment callback is being processed while it's closing.

How to reproduce

Added a variation 3 to test 0113 to reproduce the bug. The test times out.

Checklist

Please provide the following information:

• librdkafka version (release number or git tag): master
• Apache Kafka version: any
• librdkafka client configuration: any
• Operating system: any
• Critical issue

[mensfeld]

@emasab I think this works:

1. unsubscribe
2. wait so there are no assignments
3. close

With this I was not able to hang it.

[nhaq-confluent]

@emasab could this issue be closed or should it remain open?

[emasab]

@nhaq-confluent what @mensfeld proposed is a workaround, but we have to review and merge this PR I created
#4528