o Minor bugfixes (link handshake):
    - Lower the lifetime of the RSA->Ed25519 cross-certificate to
      six months, and regenerate it when it is within one month of expiring.
      Previously, we had generated this certificate at startup with
      a ten-year lifetime, but that could lead to weird behavior when
      Tor was started with a grossly inaccurate clock. Mitigates
      bug 22466; mitigation on 0.3.0.1-alpha.