o Major bugfixes (entry guards):
    - Stop trying to build circuits through entry guards for which we
      have no descriptor yet. Also, stop crashing if we *do* accidentally
      try to build a circuit in such a state. Fixes bug 21242; bugfix
      on 0.3.0.1-alpha.

  o Minor features (entry guards):
    - Do not try to build circuits until we have descriptors for our
      primary entry guards. Related to fix for bug 21242.