Make Bulk Import sidekiq workers idempotent

Currently Bulk Import sidekiq workers are not idempotent. This is not ideal, as multiple runs of such jobs can cause unexpected sideeffects, as duplicate object creation, uncaught exceptions, etc.

As per the sidekiq guidelines, such jobs should ideally be idempotent and transactional https://gitlab.com/gitlab-org/gitlab/blob/master/doc/development/sidekiq_style_guide.md#L109-109