router twins are long gone from tor. take them out of the spec.

TODO: (very soon)

      - REASON_CONNECTFAILED should include an IP.

      - Copy prose from tor-design to make everything more readable.

when do we rotate which keys (tls, link, etc)?

0. Notation:

   The port and address field denote the IPV4 address and port of the next

   onion router in the circuit; the public key hash is the SHA1 hash of the

   PKCS#1 ASN1 encoding of the next onion router's identity (signing) key.

[XXX please describe why we have this hash. my first guess is that this

way we can notice that we're already connected to this guy even if he's

connected at a different place. anything else? -RD]

   The payload for a CREATED cell, or the relay payload for an

   EXTENDED cell, contains:

   section 4.1. above, concerning choosing circIDs based on

   lexicographic order of nicknames.)

   As an extension (called router twins), if the desired next onion

   router R in the circuit is down, and some other onion router R'

   has the same public keys as R, then it's ok to extend to R' rather than R.

   When an onion router receives a CREATE cell, if it already has a

   circuit on the given connection with the given circID, it drops the

   cell.  Otherwise, after receiving the CREATE cell, it completes the