allow disabling periodic jobs

Change-Id: I106957c66c6afa9a1239833dfc8ec517e488cb61
Reviewed-on: https://gerrit.instructure.com/3734
Reviewed-by: Zach Wily <zach@instructure.com>
Tested-by: Hudson <hudson@instructure.com>

config/delayed_jobs.yml.example

@@ -9,9 +9,13 @@ production:
   # if set, workers will process this many jobs and then die, causing the pool
   # to spawn another worker. this can help return memory to the OS.
   # worker_max_job_count: 20
+  #
   # if set, workers will die and re-spawn of they exceed this memory usage
   # threshold. they will only die between jobs, not during a job.
   # worker_max_memory_usage: 1073741824
+  #
+  # disable periodic jobs auditor -- this isn't normally necessary
+  # disable_periodic_jobs: true
 
 default:
   workers:

vendor/plugins/delayed_job/lib/delayed/pool.rb

@@ -121,6 +121,8 @@ class Pool
   end
 
   def spawn_periodic_auditor
+    return if @config[:disable_periodic_jobs]
+
     @periodic_thread = Thread.new do
       # schedule the initial audit immediately on startup
       schedule_periodic_audit