Only done in -J mode because only if there's concurrency can downloading
from two remotes be faster. Without concurrency, it's likely the case that
sequential downloads from the same remote are faster than switching back
and forth between two remotes.
There is some hairy MVar code here, but basically it just keeps
the activeremotes MVar full except when deciding which remote to assign
to a thread.
Also affects gets by sync --content -J
This commit was sponsored by Jochen Bartl.
