source-backend: simplify and increase concurrency

Instead of trying to come up with some magic formula for how many
Goroutines make sense, we now exploit the fact that Goroutines are cheap
and rely on the Go runtime scheduler to make the best of the situation.

Additionally, this change should hide storage latency as good as
possible since all IO is being started at the same time.
Rackspace branding!

See this blog post for more details:
http://people.debian.org/~stapelberg/2013/12/25/codesearch-rackspace.html
add dcs-debmirror, a fast debmirror for gigabit uplinks

see the following blog post for details:
http://people.debian.org/~stapelberg/2014/01/17/debmirror-rackspace.html