cat-file resource pool

Avoid running a large number of git cat-file child processes when run with
a large -J value.

This implementation takes care to avoid adding any overhead to git-annex
when run without -J. When run with -J, there is a small bit of added
overhead, to manipulate the resource pool. That optimisation added a
fair bit of complexity.

Utility/CoProcess.hs

@@ -9,7 +9,8 @@
 {-# LANGUAGE CPP #-}
 
 module Utility.CoProcess (
-	CoProcessHandle,
+	CoProcessHandle(..),
+	CoProcessState(..),
 	start,
 	stop,
 	query,