Add dynamic deadlines to gitaly

What does this MR do?

This makes sure that the deadline we set on a gitaly call never exceeds the request deadline.

We also raise an exception if the request deadline has already been exceeded by the time we're trying to make a Gitaly call.

These deadlines don't apply when calling gitaly from Sidekiq.

We do this by storing the start time of the request in the request store on the thread (using the RequestStore).

The maximum time runtime for a request is currently set to 55 seconds.

This will make sure that we don't have to kill the unicorn if the request goes long.

Part of gitlab-com/gl-infra/scalability#71 (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