git-annex sync over tor

Mostly working!

Current todo list:

* copy --to peer seems to make the remotedaemon buffer the content in
  memory, more than I'd expect.
* update progress logs in remotedaemon send/receive
* Resuming an interrupted download fails at the end.
* 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?
* merge to master

Eventually:

* address exchange via electrum-mnemonic or magic wormhole (see PAKE)
* webapp UI for easy pairing
* friend-of-a-friend peer discovery to build more interconnected networks
  of nodes