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/Drop.hs

@@ -42,7 +42,7 @@ type Reason = String
  - The runner is used to run commands, and so can be either callCommand
  - or commandAction.
  -}
-handleDropsFrom :: [UUID] -> [Remote] -> Reason -> Bool -> Key -> AssociatedFile -> Maybe Remote -> CommandActionRunner -> Annex ()
+handleDropsFrom :: [UUID] -> [Remote] -> Reason -> Bool -> Key -> AssociatedFile -> Maybe Remote -> (CommandStart -> CommandCleanup) -> Annex ()
 handleDropsFrom locs rs reason fromhere key afile knownpresentremote runner = do
 	fs <- ifM isDirect
 		( do