Turn FallbackList into a real type, and store one in GuardMgr.

The guard manager is responsible for handing out the first hops of
tor circuits, keeping track of their successes and failures, and
remembering their states.  Given that, it makes sense to store this
information here.  It is not yet used; I'll be fixing that in
upcoming commits.

Arguably, this information no longer belongs in the directory
manager: I've added a todo about moving it.

This commit will break compilation on its own in a couple of places;
subsequent commits will fix it up.