o Minor bugfixes (bootstrapping):
    - Fetch descriptors aggressively whenever we lack enough
      to build circuits, regardless of how many descriptors we are missing.
      Previously, we would delay launching the fetch when we had fewer than
      15 missing descriptors, even if some of those descriptors were
      blocking circuits from building. Fixes bug 23985; bugfix on
      0.1.1.11-alpha. The effects of this bug became worse in 0.3.0.3-alpha,
      when we began treating missing descriptors from our primary guards
      as a reason to delay circuits.