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. Check if this is a memory leak..
* Resuming an interrupted transfer fails at the end, despite having gotten
  the whole correct file content.
* There are no error messages when things fail. Need to convert P2P runner
  from Maybe to Either String.
* update progress logs in remotedaemon send/receive
* 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 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