[BUG] the topic can not remove after the producer and consumer finished

[dockerzhang]

Describe the bug
the topic can not remove after the producer and consumer finished.

To Reproduce
1, create a partitioned topic
pulsar-admin persistent create-partitioned-topic t-kop/ns-kop/lookup2 --partitions 30
2, produce 100 message using the created topic
3, consume all produced messages.
4, delete the used topic lookup2 with pulsar command
./bin/pulsar-admin topics delete-partitioned-topic t-kop/ns-kop/lookup2
5, See error:
Failed to perform http delete request: javax.ws.rs.ClientErrorException: HTTP 412 Precondition Failed Topic has active producers/subscriptions

Expected behavior
the topic can remove after the producer and consumer finished.

Screenshots

[BewareMyPower]

Yeah, it's an existed issue for a long time because the OffsetAcker holds consumers for acknowledgement. I think there's a better way to do it.

[jiazhai]

added comments in the PR