Add jobs:check rake task to test for liveness.

This task exits with failure status if there are old jobs that have
not been picked up by workers. This can be used for monitoring.

lib/delayed/tasks.rb

@@ -22,4 +22,17 @@
       :quiet => false
     }
   end
+
+  desc "Exit with error status if any jobs older than max_age seconds haven't been attempted yet."
+  task :check, [:max_age] => :environment do |_, args|
+    args.with_defaults(:max_age => 300)
+
+    unprocessed_jobs = Delayed::Job.where('attempts = 0 AND created_at < ?', Time.now - args[:max_age].to_i).count
+
+    if unprocessed_jobs > 0
+      fail "#{unprocessed_jobs} jobs older than #{args[:max_age]} seconds have not been processed yet"
+    end
+
+  end
+
 end