Index project's associations gracefully

What does this MR do?

Currently we index our project's associations synchronically. If indexing fails we lose some data, for example gitlab-com/gl-infra/production#1499 (comment 268403773).

This MR changes the way we index project's associations, we schedule them to a separate sidekiq worker ElasticAssociationIndexerWorker which will be retrying failed indexing with different batch sizes (100,10,1) as well as retrying failed imports by queuing new sidekiq jobs with only failed associations ids.

Original issue: #195774 (closed)

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team