Improve fallback selection and output
Improve the download test:
* Allow IPv4 DirPort checks to be turned off.
* Add a timeout to stem's consensus download.
* Actually check for download errors, rather than ignoring them.
* Simplify the timeout and download error checking logic.
Tweak whitelist/blacklist checks to be more robust.
Improve logging, make it warn by default.
Cleanse fallback comments more thoroughly:
* non-printables (yes, ContactInfo can have these)
* // comments (don't rely on newlines to prevent // */ escapes)