Lay down some infrastructure for bridges in the New Guard Order.

This includes:
  * making bridge_info_t exposed but opaque
  * allowing guards where we don't know an identity
  * making it possible to learn the identity of a guard
  * creating a guard that lacks a node_t
  * remembering a guard's address and port.
  * Looking up a guard by address and port.
  * Only enforcing the rule that we need a live consensus to update
    the "listed" status for guards when we are not using bridges.