Enable threads

[jirikivaari]

Enable threads. Rails is thread-safe by design but that won't mean the app is. Rack.lock could be used for semi-threads before rest of the app is checked but Rack middleware needs to checked and frozen first.

https://devcenter.heroku.com/articles/deploying-rails-applications-with-the-puma-web-server#thread-safety-tips
https://bearmetal.eu/theden/how-do-i-know-whether-my-rails-app-is-thread-safe-or-not/

• 1. Rack middleware safety
• 2. Use Rack.lock for pseudo-thread-safety. Can be removed after the
• 3. Rails safety?
• 4. Gem safety?
• 5. Something else?

Currently using puma workers to get more concurrency.