Merge branch 'maint-0.2.9'

  o Minor bugfixes (consensus downloads):

    - If a consensus expires while we are waiting for certificates to download,

      stop waiting for certificates.

    - If we stop waiting for certificates less than a minute after we started

      downloading them, do not consider the certificate download failure a

      separate failure.

      Fixes bug 20533; bugfix on commit e0204f21 in 0.2.0.9-alpha.

  o Minor bugfixes (directory download scheduling):

    - Remove the maximum delay on exponential-backoff scheduling.

      Since we now allow an infinite number of failures (see ticket

      20536), we must now allow the time to grow longer on each failure.

      Fixes bug 20534; bugfix on 0.2.9.1-alpha.

  o Major bugfixes (download scheduling):

    - When using an exponential backoff schedule, do not give up on

      dowloading just because we have failed a bunch of times.  Since

      each delay is longer than the last, retrying indefinitely won't

      hurt. Fixes bug 20536; bugfix on 0.2.9.1-alpha.

  o Minor bugfixes (download timing):

    - When determining when to download a directory object, handle times

      after 2038 if the operating system supports that.  (Someday this will be

      important!)  Fixes bug 20587; bugfix on 0.2.8.1-alpha.

  o Minor bugfixes (relay bootstrap):

    - Ensure relays don't make multiple connections during bootstrap.

      Fixes bug 20591; bugfix on 0.2.8.1-alpha.