get, move, copy, mirror: Concurrent downloads and uploads are now supported!

This works, and seems fairly robust. Clean get of 20 files at -J3. At -J10,
there are some messages about ssh multiplexing, probably due to a race
spinning up the ssh connection cacher. But, it manages to get all the files
ok regardless.

The progress bars are a scrambled mess though, due to bugs in
ascii-progress, which I've already filed. Particularly this one:
https://github.com/yamadapc/haskell-ascii-progress/issues/8

Annex.hs

@@ -68,6 +68,7 @@ import Utility.Url
 
 import "mtl" Control.Monad.Reader
 import Control.Concurrent
+import Control.Concurrent.Async
 import qualified Data.Map as M
 import qualified Data.Set as S
 
@@ -133,6 +134,7 @@ data AnnexState = AnnexState
 #endif
 	, existinghooks :: M.Map Git.Hook.Hook Bool
 	, desktopnotify :: DesktopNotify
+	, workers :: [Either AnnexState (Async AnnexState)]
 	}
 
 newState :: GitConfig -> Git.Repo -> AnnexState
@@ -178,6 +180,7 @@ newState c r = AnnexState
 #endif
 	, existinghooks = M.empty
 	, desktopnotify = mempty
+	, workers = []
 	}
 
 {- Makes an Annex state object for the specified git repo.