Bug 959915 - Part 1: implement computeClientState.
Bug 959915 - Part 2: compute and return client state from Married.
Bug 959915 - Part 3: extend TokenServerClient to use X-Client-State header.
Review comment: comment and validate in computeClientState.
Review comment: extract TokenFetchResourceDelegate.
Review comment: an awful hacky test for X-Client-State header.
Follow-up: update test URLs.
Follow-up: add endpoint allocation test.
Bug 959915 - Adapt FxA token server client to include X-Client-State header. r=nalexander