Complete & commit current task(s) on rebalance

[francoisferrand]

Implement rebalance callback, so that we will now properly handle the rebalance process at
application level, and thus avoid extra rebalances or processing the same entry multiple times:

• On unassign, wait until current tasks are complete, by draining the queue;
• Then explicitely commit, to ensure the persisted timely (since we are usually relying on a
  auto-commit timer, which may happen at any time in the process)
• Finally call the unassign() method of consumer to let the cluster sync and proceed with the
  rebalance.

We are expected (by kafka) to do this whole process within max.poll.interval.ms (default 5min),
similar to how we are expected to poll periodically. This should be fine in general, except in case
of a bug where a task is really stuck. In that case, the consumer will get kicked out of the
cluster, and commit may fail. If tasks eventually unblock, the consumer will reconnect to the
cluster on next call to consume(), which will trigger a rebalance. There is no guard yet against
this, and we keep the same behavior as before in those cases.

• Store offset in BackbeatConsumer bootstrap
• Add kafka consumer rebalance test
• Drain the queue on rebalance
• Commit before unassigning
• Fix BackbeatConsumer breakbeat tests

Issue: BB-441

[bert-e]

Hello francoisferrand,

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Status report is not available.

[bert-e]

Request integration branches

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[fmonjalet]

No question to ask in the end, the code answered it itself!

[francoisferrand]

/approve

[bert-e]

Integration data created

I have created the integration data for the additional destination branches.

• this pull request will merge bugfix/BB-441 into
  development/8.6
• w/8.7/bugfix/BB-441 will be merged into development/8.7

The following branches will NOT be impacted:

• development/7.10
• development/7.4
• development/7.70
• development/8.5

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@bert-e create_pull_requests

The following options are set: approve

[bert-e]

In the queue

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branch(es) as soon as builds have passed.

The changeset will be merged in:

• ✔️ development/8.6

• ✔️ development/8.7

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• development/7.10
• development/7.4
• development/7.70
• development/8.5

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The following options are set: approve

[bert-e]

I have successfully merged the changeset of this pull request
into targetted development branches:

• ✔️ development/8.6

• ✔️ development/8.7

The following branches have NOT changed:

• development/7.10
• development/7.4
• development/7.70
• development/8.5

Please check the status of the associated issue BB-441.

Goodbye francoisferrand.