Update epic cache counts when issue is destroyed

What does this MR do and why?

When an issue is deleted, update of epic cache counts was not scheduled because in after_commit callback reference to issue's epic is already deleted. For this reason in case of issue destroy we store referenced epic_id before destroy is done and schedule cache update for this epic in after_commit.

Didn't add a changelog because these cached values are not used yet.

Related to #387671 (closed)

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

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