avoid STM deadlock

When all worker threads are running and enteringStage is called,
it waits for an idle slot. If all off the other threads then call it in
turn, a deadlock occurrs.

This is the same problem I didn't actually fix in
5a9842d7ed.

Fixed by doing two separate STM transactions, the first replaces its
active thread with an idle thread, and the second waits for another idle
thread. That guarantees there will eventually be an idle thread to find.

The changes to WorkerPool were necessary because it can't add an idle
thread containing the Annex state and go on to run an action using that
same state, so I had to remove the Annex state from IdleWorker.

CmdLine/Action.hs

@@ -129,8 +129,8 @@ finishCommandActions = Annex.getState Annex.workers >>= \case
 	Nothing -> noop
 	Just tv -> do
 		Annex.changeState $ \s -> s { Annex.workers = Nothing }
-		WorkerPool _ l <- liftIO $ atomically $ takeTMVar tv
-		forM_ (mapMaybe workerAsync l) $ \aid ->
+		pool <- liftIO $ atomically $ takeTMVar tv
+		forM_ (mapMaybe workerAsync $ workerList pool) $ \aid ->
 			liftIO (waitCatch aid) >>= \case
 				Left _ -> noop
 				Right st -> mergeState st