Separate the various roles that directory authorities play, from a configuration POV

It would be handy if the following roles were split up:

1. The list of IP:Orport:Identity to which every relay should upload every descriptor.
2. The list of IP:Orport:Identity from which caches should expect to find canonical consensuses and descriptors.
3. The list of IP:Orport:Identity from which non-caches should expect to bootstrap consensuses and descriptors. (See 'fallbackdir')
4. The list of keys that must sign a vote or a consensus.
5. The list of IP:Orport:Identity that authorities use when sending and receiving votes.

Splitting roles up in this way would better prepare us for an implementation of prop#257 down the road.