Inherit {start,end}_date from child epics or milestones

Epic would now inherit start_date or end_date from its
related issues milestone dates or from child epic
depending on which one gives the wider time spread.

Uses a single update statement to update start_date, due_date,
start_date_sourcing_milestone_id, start_date_sourcing_epic_id,
due_date_sourcing_milestone_id, due_date_sourcing_epic_id

Epic inherited dates update for multiple epics moved to
an async worker

Update epic dates in batches

Add foreign key constraints and indexes on
start_date_sourcing_epic_id and due_date_sourcing_epic_id

https://gitlab.com/gitlab-org/gitlab-ee/issues/7332