add batch writes in cache.

In this PR, batch writes is added to the in memory cache as mentioned in #182
I am not too happy with memory duplication. We are doing this because the current storage driver interface does not support all requirements. On the other hand, the additional memory usage might be negligible in reality. I wonder that @rjnagal and @vmarmol think about this.

If we care about memory, we can refactor the storage driver abstraction, or better move non-memory storage drivers out of cAdvisor completely.