Merge branch 'actioncable-dev-config' into 'master'

Start only 1 worker for ActionCable in development

See merge request gitlab-org/gitlab!29314

config/puma_actioncable.example.development.rb

@@ -43,7 +43,7 @@ queue_requests false
 # accepted protocols.
 bind 'unix:///home/git/gitlab_actioncable.socket'
 
-workers 2
+workers 1
 
 require_relative "/home/git/gitlab/lib/gitlab/cluster/lifecycle_events"