[sheepdog] latest snapshot object reclaim feature

[Hitoshi Mitake]

Hi Valerio, Alessandro, Kazutaka-san,

I altered the snapshot object reclaim feature and pushed the new code
to the below branch:

https://github.com/sheepdog/sheepdog/tree/snapshot-object-reclaim

Test scripts and documents are not updated yet. And I need to support
hypervolume later. But extensys folks can use current code for
evaluation.

I'll clean the remaining todos as soon as possible.

Thanks,
Hitoshi