Fallback directory v2 parsing

Adding support for the proposed v2 fallback directory format...

  https://lists.torproject.org/pipermail/tor-dev/2017-December/012721.html

This isn't live yet and I won't be checking persistance support until it is,
but good starting point. Remaining thoughs are...

  1. The 'extrainfo=' line is optional but we're using it as a delimiter, so
     this won't work if it's omitted.

  2. It would be nice if the document explicitly said its format version.

  3. I suspect reading persisted has_extrainfo attributes mistakenly provide a
     str rather than bool. As mentioned above I'll be checking persistence once
     it's live.