Using 'GETINFO md/all' to get microdescriptors if available

Thanks to rl1987 tor now has a GETINFO method to retrieve microdescriptors
(similar to 'GETINFO desc/all-recent', 'GETINFO ns/all', etc).

  https://gitweb.torproject.org/torspec.git/commit/?id=b5396d5
  https://trac.torproject.org/projects/tor/ticket/26210