Allow to enable Puma single for GCK and GDK

In the past we were running Unicorn to have a multiple workers (at least two) as required to processes some requests.

Since we run Puma, we can live with running multiple threads instead of multiple workers to achieve the same, and reduce amount of used memory for development environments.

I would propose to enable puma single for GDK and GCK with setting by default to use 4 threads. This will reduce a lot of memory for these installations (cluster+2 workers would use around 400MB more) without likely a negatively impacting running application for development purposes.

Summary

We decided that this is not advised to enable it by default. We will allow this as an option.