Detect and report stalled directory downloads

Our current bootstrap-progress code warns about problems with connecting to the network, but not problems with fetching directory information. This branch updates tor-dirmgr and arti-client to report this information.

Closes #468 (closed).

The branch also does some refactoring on tor-dirmgr's status-reporting logic. The old code had a few kludges which it made sense to iron out before working on this feature.