cleanup failed delayed_jobs by default (for table speed)

config/initializers/delayed_job.rb

@@ -1,4 +1,4 @@
-Delayed::Worker.destroy_failed_jobs = false
+Delayed::Worker.destroy_failed_jobs = true
 Delayed::Worker.max_attempts = 5
 Delayed::Worker.max_run_time = 20.minutes
 Delayed::Worker.default_priority = 10