Add helper to cleanup delayed sidekiq timechunks

[Drowze]

For context:

#869 added the delayed_sidekiq strategy, which by design will only create a sidekiq worker to update an index every N seconds (10 seconds by default).
The implementation details for that involves redis sorted sets: it creates a sorted set for each index that uses the strategy and adds a new member to it everytime we call reindex (with the weight being calculated from the current timestamp, generating a unique number every N seconds).

The problem:

Since we're writing directly to Redis, if we use this strategy in tests we're subject to state leaking and creating flaky tests. For example:

• test A runs some code that calls MyClass.import!(1, strategy: :delayed_sidekiq) and not flush the Sidekiq workers or the redis database
• in less than 10 seconds, test B runs some code that calls MyClass.import!(1, strategy: :delayed_sidekiq), flushes Sidekiq workers and expects the index to be updated.

On the above scenario, test B will fail as no worker will be enqueued since import! was called within 10 seconds in between the tests (also note that Sidekiq runs in memory by default when running tests, so workers enqueued by test A are not present in test B).

The fix:

This PR adds a way to workaround the issue by cleaning up the saved timechunks. It can easily be added to e.g. rspec hooks via:

RSpec.configure do |config|
  config.after { Chewy::Strategy::DelayedSidekiq.clear_timechunks! }
end

---

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• The PR relates to only one subject with a clear title and description in grammatically correct, complete sentences.
• Wrote good commit messages.
• Commit message starts with [Fix #issue-number] (if the related issue exists).
• Feature branch is up-to-date with master (if not - rebase it).
• Squashed related commits together.
• Added tests.
• Added an entry to the changelog if the new code introduces user-observable changes. See changelog entry format for details.

[konalegi]

Could you please rebase PR?

[konalegi]

hey @Drowze , could you please rebase your PR?

[Drowze]

Hey @konalegi, sorry for the delay! I didn't see the notification for the first message 😅

Just rebased this PR! 👍

[konalegi]

Hey, unfortunately rubocop is failing :(
I suppose it's because we recently switched from ruby 2.x to 3.x syntax.

[Drowze]

I see! Just fixed the rubocop issues and updated the PR 😄