When using an external special remote with -Jn, multiple transfers do not
happen concurrently to the remote. A single process is
started for the external special remote, and there's locking to only let
one request be made of it at a time.

This should not be hard to make to use a pool of Externals, starting up a
new one if the pool is empty or all in use. --[[Joey]]

[[users/yoh]] may want this for datalad?

> [[done]] --[[Joey]]