disentangle concurrency and message type

This makes -Jn work with --json and --quiet, where before
setting -Jn disabled those options.

Concurrent json output is currently a mess though since threads output
chunks over top of one-another.

Types/Concurrency.hs

@@ -0,0 +1,8 @@
+{- Copyright 2016 Joey Hess <id@joeyh.name>
+ -
+ - Licensed under the GNU GPL version 3 or higher.
+ -}
+
+module Types.Concurrency where
+
+data Concurrency = NonConcurrent | Concurrent Int