  o Minor bugfixes:
    - Use direct writes rather than stdio when building microdescriptor
      caches, in an attempt to mitigate bug 8031, or at least make it
      less common.
    - Warn more aggressively when flushing microdescriptors to a
      microdescriptor cache fails, in an attempt to mitegate bug 8031,
      or at least make it more diagnosable.
