Realized recently that despite all the nice concurrency support in
git-annex, external special remotes were limited to handling one request at
a time.

While the external special remote prococol could almost support concurrent
requests, that would complicate implementing them, and probably need a
version flag to enable to avoid breaking existing ones.

Instead, made git-annex start up multiple external special remote processes
as needed to handle concurrency.

Today's work was sponsored by Josh Taylor on Patreon.
