[Ruby] Add rage-rb

[rsamoilov]

Rage is a fast web framework compatible with Rails. It uses an event-driven architecture and implements a lightweight, cooperative concurrency model based on Ruby Fibers.

https://github.com/rage-rb/rage

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I'm not sure I've worked out the size of the DB pool correctly. It's possible I will need to reduce it in case there are errors under high concurrency. But before that, I'd like to see how it performs on your setup with these settings.