Update epic cache counts on project/group deletion

What does this MR do and why?

When issue's count or weight is changed for an epic, we update its cached values. These cached values are sum of both direct epic's issues and also sub-epic's issues.

Because we want to include subepics weights/counts too, when cached value is changed we propagate this change to all epic ancestors. This is achieved by scheduling cache update for epic's parent in epic's after_commit callback.

Related to #356683 (closed)

DB changes:

• queries to get impacted epics when destroying group or project - !90615 (comment 1000841520) and !90615 (comment 1000999580)
• queries to get epic's cached sum/counts - !90615 (comment 1004585335) and !90615 (comment 1004589231)

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ee/app/models/ee/project.rb

@@ -906,14 +906,17 @@ def epic_ids_referenced_by_issues
         result[epic_issue.issue_id] = epic_issue.epic_id
       end
 
-      ids = Set.new
-      issues.each_batch(of: 100) do |batch|
+      scope = issues.order(:iid)
+      iterator = ::Gitlab::Pagination::Keyset::Iterator.new(scope: scope)
+      epic_ids = Set.new
+
+      iterator.each_batch(of: 500) do |batch|
         batch.pluck(:id).each do |issue_id|
-          ids << epic_by_issue[issue_id] if epic_by_issue.has_key?(issue_id)
+          epic_ids << epic_by_issue[issue_id] if epic_by_issue.has_key?(issue_id)
         end
       end
 
-      ids
+      epic_ids
     end
 
     private