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

CmdLine/GitAnnex/Options.hs

@@ -138,6 +138,14 @@ jsonOption :: Option
 jsonOption = Option ['j'] ["json"] (NoArg (Annex.setOutput JSONOutput))
 	"enable JSON output"
 
+jobsOption :: Option
+jobsOption = Option ['J'] ["jobs"] (ReqArg set paramNumber)
+	"enable concurrent jobs"
+  where
+	set s = case readish s of
+		Nothing -> error "Bad --jobs number"
+		Just n -> Annex.setOutput (ParallelOutput n)
+
 timeLimitOption :: Option
 timeLimitOption = Option ['T'] ["time-limit"]
 	(ReqArg Limit.addTimeLimit paramTime)