tor-dirmgr/state.rs: take an object to get a netdir, not a netdir

- Taking a previous netdir directly and keeping it around before we need
  it is a bit of a waste of memory, and also doesn't mesh well with how
  SharedMutArc works.
- To remedy this, introduce a new trait `PreviousNetDir` and have the
  state machines take that instead. (I was a bit tempted to just pass in
  the SharedMutArc directly. Maybe I should've done that.)