Pending acks uses lots of heap memory in consumer

[codelipenghui]

Describe the bug

As the image shows, a consumer uses more than 2MB heap memory to maintain the pending acks.
To Reproduce
Steps to reproduce the behavior:
This problem can reproduce when doing some performance tests and especially when the batch is disabled.

Expected behavior
Reduce the heap memory usages for the pending ack of the consumer

Additional context
Pulsar 2.6.0

[tisonkun]

Closed as stale. Please create a new issue if it's still relevant to the maintained versions.