directory special remote: Made more efficient and robust.

Files are now written to a tmp directory in the remote, and once all
chunks are written, etc, it's moved into the final place atomically.

For now, checkpresent still checks every single chunk of a file, because
the old method could leave partially transferred files with some chunks
present and others not.

Command/SendKey.hs

@@ -12,7 +12,6 @@ import Command
 import Annex.Content
 import Utility.Rsync
 import Logs.Transfer
-import Types.Remote
 import qualified Fields
 
 def :: [Command]