The node_t type is meant to serve two key functions:

  1) Abstracting difference between routerinfo_t and microdesc_t
     so that clients can use microdesc_t instead of routerinfo_t.

  2) Being a central place to hold mutable state about nodes
     formerly held in routerstatus_t and routerinfo_t.

This patch implements a nodelist type that holds a node for every
router that we would consider using.