Add max_request_duration config to gitlab_rails

This option lets the rails app know how much time it has to process a request. And make decisions accordingly.

By default the time is 90% of the timeout configured for the worker. This gives us some time during the request to clean up if we need to.

Setting a duration higher than the worker timeout will raise an error.

This is required for gitlab-com/gl-infra/scalability#71 (closed)