Only check request_deadline flag once per request

What does this MR do?

Every time we check a feature flag using Feature.enabled? it could return a different value based on the percentage that it's enabled for.

So making sure we only call this value once per request, so we have a consistent state for the entire request.

Closes gitlab-com/gl-infra/scalability#104 (closed)

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• [-] Code review guidelines
• [-] Merge request performance guidelines
• [-] Style guides
• [-] Database guides
• [-] Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers