[WIP] Making AddStats() accept multiple parameters.

This PR follows up the discussion on #200. It changes the StorageDriver interface so that ``AddStats()`` will accept multiple stats making the underlying implementation possible to do batch writes.
I have same question. We could have two choices:

- ``AddStats(ref info.ContainerReference, stats ...*info.ContainerStats) error``: This introduced least change and compatible with old interface. However, it could not maximize the benefit of batch writes: If there're multiple containers, it has to be called several times.
- ``AddStats(pairs ...storage.ContainerRefStatsPair) error``: This could let the storage driver to one single write operation for all containers with several stats. However, this requires more changes in the old code.

Since we need to change the code anyway, I chose the second option.