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..
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.
