Sessions are failing to renew, because used request headers
  are set on the TwitterSession instance, but never reset.
  Thus, trying to login_guest() with an outdated X-Guest-Token
  will make the activate.json request for the new token be rejected
  because of that outdated token.

  Solution: Reset TwitterSession._headers in renew_session()
            to just have the User-Agent value before trying to login.

  Introduces new option --timeline-termination-endpoint
  Set this to the URI of the TimelineTermination testing service,
  e.g. http://ttb.intern/api/v1/test/ (trailing slash!)