backport the new authority and fallback file formats
After doing legacy/trac#24851 and legacy/trac#24852.
* backport them to all supported tor releases:
* authorities: ~~0.2.5 (EOL 1 May 2018),~~ 0.2.9, ~~0.3.0 (EOL 1 Feb 2018),~~ 0.3.1, 0.3.2, 0.3.3
* fallbacks: 0.2.9, ~~0.3.0 (EOL 1 Feb 2018),~~ 0.3.1, 0.3.2, 0.3.3
* make sure all supported tor releases parse the files correctly (unit tests)
issue