git-annex sync over tor

Mostly working!

Current todo list:

* When a transfer can't be done because another transfer of the same 
  object is already in progress, the message about this is output by the
  remotedaemon --debug, but not forwarded to the peer, which shows
  "Connection reset by peer"
* Think about locking some more. What happens if the connection to the peer
  is dropped while we think we're locking content there from being dropped?

Eventually:

* Limiting authtokens to read-only access.
* Revoking authtokens. (This and read-only need a name associated with an
  authtoken, so the user can adjust its configuration after creating it.)
* address exchange for peering. See [[design/assistant/telehash]].
* Webapp UI to set it upt.
* friend-of-a-friend peer discovery to build more interconnected networks
  of nodes
* Discovery of nodes on same LAN, and direct connection to them.