Unicorn can receive requests long after ProxyHeadersTimeout

Workhorse fails requests that take Unicorn more than 5 minutes to respond to. This is a backstop; Unicorn should fail after 1 minute anyway.

Oddly, Unicorn (gitlab-rails) is logging requests that spent much longer than 5 minutes in limbo. How is this possible?

Follow-up to discussion in https://gitlab.com/gitlab-com/infrastructure/issues/4380#note_81250944

cc @andrewn