Logic to implement rendezvous/introduction via unknown servers.

Logic to implement rendezvous/introduction via unknown servers.

- Add a new extend_info_t datatype to hold information needed to
  extend a circuit (addr,port,keyid,onion_key).  Use it in cpath and
  build_state.  Make appropriate functions take or return it instead of
  routerinfo_t or keyid.

- #if 0 needless check in circuit_get_by_edge_conn; if nobody triggers this
  error in 0.1.0.10, nobody will trigger it.

- Implement new hidden service descriptor format, which contains "extend
  info" for introduction points, along with protocol version list.
  - Parse new format.
  - Generate new format
  - Cache old and new formats alongside each other.
  - Directories serve "old" format if asked in old way, "newest available"
    format if asked in new way.
  - Use new format to find introduction points if possible; otherwise fall
    back.  Keep nickname lists and extendinfo lists in sync.
  - Tests for new format.

- Implement new "v2" INTRODUCE cell format.
  - Accept new format
  - Use new format if we have a versioned service descriptor that says the
    server accepts the new format.

- Add documentation for functions and data types.

svn:r4506