Have directory authorities vote on IPv6 OR ports according to the spec

Define new new consensus method 14 adding "a" lines to vote and
consensus documents.

From proposal 186:

  As with other data in the vote derived from the descriptor, the
  consensus will include whichever set of "a" lines are given by the
  most authorities who voted for the descriptor digest that will be
  used for the router.

This patch implements this.