BURs: process bulk updates in dedicated job queue.

Add a dedicated queue for bulk update requests and process it using a
single worker. This prevents bulk updates from consuming all available
workers and preventing other job types from running.

This also effectively serializes bulk updates so that they're processed
one-at-a-time instead of in parallel. This will be slower overall but
may avoid some of the issues with indeterminate update order under
parallel updates.

lib/capistrano/tasks/delayed_job.rake

@@ -4,6 +4,8 @@ namespace :delayed_job do
     on roles(:worker) do
       within current_path do
         with rails_env: fetch(:rails_env) do
+          execute :"systemd-run", "--user --collect --slice delayed_job --unit delayed_job.bulk_update -E RAILS_ENV=$RAILS_ENV -p WorkingDirectory=$PWD -p Restart=always #{bundle} exec script/delayed_job --queues=bulk_update run"
+
           fetch(:delayed_job_workers, 16).times do |n|
             bundle = SSHKit.config.command_map[:bundle]
             execute :"systemd-run", "--user --collect --slice delayed_job --unit delayed_job.#{n} -E RAILS_ENV=$RAILS_ENV -p WorkingDirectory=$PWD -p Restart=always #{bundle} exec script/delayed_job --queues=default run"