The `DirBootstrapStatus` type now exposes a blockage() method to
return an `Option<DirBlockage>`.
The blockage types reported are more low-level than I'd like, but
they are IMO good enough for now: we'll want to get experience with
actual vs hypothetical problems before we refine them.
This change also means that we need to create the handle and scheduler
earlier in the process of creating the DirMgr. If we don't, we won't
have a way to manage the task before bootstrap() returns.