kgo: add CloseAllowingRebalance

[twmb]

Two forks of this repo added logs all over to track down why Close was hanging:

ar3s3ru/kprocessor@b5b4b3d#r76473229
bernardoVale@29aff20#commitcomment-78931790

The main issue is that BlockRebalanceOnPoll also blocks client closing, because client close leaves a group, which causes a rebalance.

The client shouldn't just unblock when closing, because the user may have some logic that is unsafe to run unless guaranteed to be serial.

At a minimum, the docs on Close need to be updated to note how the user needs to run AllowRebalance before closing. Potentially, another API can be added, CloseAllowingRebalance.

[twmb]

This has been released in v1.7.0