Don't use stale offset information when consuming messages (#174)
Don't use stale offset information when consuming messages (#174)

* Don't use stale offset information when consuming messages

After committing offsets, the consumer would clear the information of
which offsets had been processed but fail to refresh the broker supplied
table, leading to erroneous offset information.

* Only clear consumer offsets when re-joining a group

Clearing on every commit is wasteful.

zendesk/ruby-kafka,crashlytics/ruby-kafka,zendesk/ruby-kafka,crashlytics/ruby-kafka