Adjust number of Sidekiq processes for GitLab

By default, GitLab starts 25 processes of Sidekiq. If this certainly makes sense on systems with high load, this could be overkill for simple instances of GitLab with only a handful of users and active projects. To reduce memory usage, it is possible to reduce the number of concurrent processes for Sidekiq by adding the following line to the /path/to/gitlab/config/sidekiq_queues.yml file:

:concurrency: 2