Async job for note + event when creating/updating an EpicIssue link

What does this MR do?

When issue's epic is assigned or changed, this operation is SQL-expensive. We can do part at least a small part of logic asynchronously which saves us some SQL queries:

• creation of system notes - this saves us noticable amount SQL queries
• usage ping metrics update - this is actually pretty cheap, but still no reason not to do it on background if we have the worker

It's questionable whether it's worth to offload system notes creation into an async worker as it adds some extra logic, on the other side I think some services (in this case issue update service) became pretty heavy and expensive. Given the fact that requests should do <100 SQL queries and system notes creation in this case takes 25 queries (overall count including savepoints, commits...), I would say it is still worth it.

Related to #247861 (closed)

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

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• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
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• This change is backwards compatible across updates, or this does not apply.

Availability and Testing

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Security

Does this MR contain changes to processing or storing of credentials or tokens, authorization and authentication methods or other items described in the security review guidelines? If not, then delete this Security section.

• 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