Otherwise one cannot create two containers with different releases
(let's say saucy [cached] and raring [not caced]) if both are not
cached on the local filesystem already. The lock blocks cached
one to move forward until not cached one finishes it's downloads.

Fix that by seperating locks using release names
Signed-off-by: S.Çağlar Onur <caglar@10ur.org>
Acked-by: Stéphane Graber <stgraber@ubuntu.com>