Checkpoint in-progress fixes:

Checkpoint in-progress fixes:
Add 'testing' circuit purpose, for reachability testing.
Notice when our IP changes, and reset stats.
Try to pull down a directory via Tor to see if our DirPort is working.
Try to extend a circuit back to us to see if our ORPort is working.
Only publish a descriptor if they're both reachable.

These mostly work, and I'd better get them in before I cause conflicts.

svn:r3703