Change shrinker to release oldest data first

Removing prev also makes the code simpler.

Signed-off-by: Raphael S. Carvalho <raphaelsc@cloudius-systems.com>
Dealing with concurrency

Signed-off-by: Raphael S. Carvalho <raphaelsc@cloudius-systems.com>
Merge pull request #4 from raphaelsc/concurrency

Concurrency