Limited amount of pruned Event rows per run

Old deployments of Gitlab might have a big number of old events to be
deleted. Such numbers cause the worker to timeout.
I've limited the amount of rows that should be destroyed at once to
10000, and increased how often pruning shall take place to 4 times a
day.

CHANGELOG

@@ -3,6 +3,7 @@ Please view this file on the master branch, on stable branches it's out of date.
 v 8.12.0 (unreleased)
   - Add ability to fork to a specific namespace using API. (ritave)
   - Cleanup misalignments in Issue list view !6206
+  - Prune events older than 12 months. @ritave <olaf@tomalka.me>
   - Prepend blank line to `Closes` message on merge request linked to issue (lukehowell)
   - Prune events older than 12 months.
   - Filter tags by name !6121

app/workers/prune_old_events_worker.rb

@@ -3,6 +3,6 @@ class PruneOldEventsWorker
   def perform
     # Contribution calendar shows maximum 12 months of events
-    Event.where('created_at < ?', (12.months + 1.day).ago).destroy_all
+    Event.delete(Event.unscoped.where('created_at < ?', (12.months + 1.day).ago).limit(10_000).pluck(:id))
   end
 end

config/initializers/1_settings.rb

@@ -300,7 +300,7 @@ Settings.cron_jobs['remove_expired_group_links_worker'] ||= Settingslogic.new({}
 Settings.cron_jobs['remove_expired_group_links_worker']['cron'] ||= '10 0 * * *'
 Settings.cron_jobs['remove_expired_group_links_worker']['job_class'] = 'RemoveExpiredGroupLinksWorker'
 Settings.cron_jobs['prune_old_events_worker'] ||= Settingslogic.new({})
-Settings.cron_jobs['prune_old_events_worker']['cron'] ||= '0 0 * * *'
+Settings.cron_jobs['prune_old_events_worker']['cron'] ||= '* */6 * * *'
 Settings.cron_jobs['prune_old_events_worker']['job_class'] = 'PruneOldEventsWorker'
 #