check-attr resource pool

Limited to min of -JN or number of CPU cores, because it will often be
CPU bound, once it's read the gitignore file for a directory.

In some situations it's more disk bound, but in any case it's unlikely
to be the main bottleneck that -J is used to avoid. Eg, when dropping,
this is used for numcopies checks, but the main bottleneck will be
accessing the remotes to verify presence. So the user might decide to
-J32 that, but having 32 check-attr processes would just waste however
many filehandles they open, and probably worsen their performance due to
CPU contention.

Note that, I first tried just letting up to the -JN be started. However,
even when it's no bottleneck at all, that still results in all of them
being started. Why? Well, all the worker threads start up nearly
simulantaneously, so there's a thundering herd..

Utility/ResourcePool.hs

@@ -8,7 +8,7 @@
 {-# LANGUAGE BangPatterns #-}
 
 module Utility.ResourcePool (
-	ResourcePool,
+	ResourcePool(..),
 	mkResourcePool,
 	mkResourcePoolNonConcurrent,
 	withResourcePool,