backport the new authority and fallback file formats

After doing legacy/trac#24851 (moved) and legacy/trac#24852 (moved).

• 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)