Regression due to fix for #313

[patkivikram]

Checklist

• [ x] I have included information about relevant versions
• [ x] I have verified that the issue persists when using the master branch of Faust.

Steps to reproduce

When a compacted topic with gaps is processed by an agent, the consumer never commits any offsets

Expected behavior

The consumer should identify gaps in the topic and acknowledge messages

Actual behavior

The consumer stop committing offets. This is likely due to #313

Versions

• Python version 3.8
• Faust version 0.8.6
• Operating system Centos

[wbarnha]

I'm surprised our unit tests didn't pick this up. It'd be nice to add another unit test with this to make sure the new change works as intended.