KafkaIO doesn't commit offsets while being used as bounded source

[kennknowles]

While using KafkaIO as bounded source (with withMaxReadTime() or withMaxNumRecords()) it seems doesn't commit offsets all the time.

See the details in the discussion on user@list.

Imported from Jira BEAM-6466. Original Jira may contain additional context.
Reported by: aromanenko.