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 request duration defaults to 55 seconds, as the default
worker timeout is 60 seconds. But can be configured through
gitlab.yml.