Improved exit policy syntax; basic client-side DNS caching.

- Exit policies now support bitmasks (18.0.0.0/255.0.0.0) and bitcounts
  18.0.0.0/8.  Policies are parsed on startup, not when comparing to them.

- desired_path_len is now part of an opaque cpath_build_state_t structure.

- END_REASON_EXITPOLICY cells no longer include a port.

- RELAY_COMMAND_CONNECTED cells now include the IP address we've connected
  to.

- connection_edge now has a client_dns cache to remember resolved addresses.
  It gets populated by RELAY_COMMAND_CONNECTED cells and END_REASON_EXITPOLICY
  cells.  It gets used by connection_ap_handshake_send_begin.  We don't
  compare it to exit policies yet.


svn:r812