Allow PUMA_SINGLE_MODE=true to start a non-threaded puma instance

What does this Merge Request do and why?

Allows the environment variable PUMA_SINGLE_MODE to be set. This indicates that puma should be started in single mode, running only one thread. This allows for single-threaded debugging using binding.pry. If the number of workers and threads are not reduced to the minimum, then requests will continue to run even when stopped in pry. This returns the ability we had with the original idk thin.

gdk stop rails-web && GITLAB_RAILS_RACK_TIMEOUT_ENABLE_LOGGING=false PUMA_SINGLE_MODE=true gdk rails s

Merge Request checklist

• This change is backward compatible. If not, please include steps to communicate to our users.
• Tests added for new functionality. If not, please raise Issue to follow-up.
• Documentation added/updated, if needed.
• gdk doctor test added, if needed.
• Add the ~highlight label if this MR should be included in the CHANGELOG.md.

Closes #1592 (closed)