start splitting out readonly values from AnnexState

Values in AnnexRead can be read more efficiently, without MVar overhead.
Only a few things have been moved into there, and the performance
increase so far is not likely to be noticable.

This is groundwork for putting more stuff in there, particularly a value
that indicates if debugging is enabled.

The obvious next step is to change option parsing to not run in the
Annex monad to set values in AnnexState, and instead return a pure value
that gets stored in AnnexRead.

Annex/Concurrent.hs

@@ -55,9 +55,10 @@ setConcurrency' c f = do
  -}
 forkState :: Annex a -> Annex (IO (Annex a))
 forkState a = do
+	rd <- Annex.getRead id
 	st <- dupState
 	return $ do
-		(ret, newst) <- run st a
+		(ret, (newst, _rd)) <- run (st, rd) a
 		return $ do
 			mergeState newst
 			return ret
@@ -90,7 +91,9 @@ dupState = do
  - Also closes various handles in it. -}
 mergeState :: AnnexState -> Annex ()
 mergeState st = do
-	st' <- liftIO $ snd <$> run st stopNonConcurrentSafeCoProcesses
+	rd <- Annex.getRead id
+	st' <- liftIO $ (fst . snd)
+		<$> run (st, rd) stopNonConcurrentSafeCoProcesses
 	forM_ (M.toList $ Annex.cleanupactions st') $
 		uncurry addCleanupAction
 	Annex.Queue.mergeFrom st'