o Infrastructure features:
    - Introduce new channel_t abstraction between circuits and or_connection_t
      to allow for implementing alternate OR-to-OR transports.  A channel_t is
      an abstract object which can either be a cell-bearing channel, which is
      responsible for authenticating and handshaking with the remote OR and
      transmitting cells to and from it, or a listening channel, which spawns
      new cell-bearing channels at the request of remote ORs.

    - Also new is the channel_tls_t subclass of channel_t, adapting it to the
      existing or_connection_t code.  The V2/V3 protocol handshaking code
      which formerly resided in command.c has been moved below the channel_t
      abstraction layer and may be found in channeltls.c now.