Add the `max_request_duration` configuration option

In gitlab-org/omnibus-gitlab!3830 (merged) we've added a max_request_duration configuration option to GitLab-rails (gitlab.yml).

In omnibus, we're using the worker_timeout set in unicorn or puma, to calculate a default for max_request_duration and set it to 95% of the worker timeout. If a different value is configured in gitlab.rb that is used, if it is higher than the configured worker timeout, an error is raised during reconfigure.

If this setting is not present, gitlab-rails will default to 57 seconds, which is the 95% of the default 60s worker timeout.