Storage backend for stats

This PR is trying to build a starting point for #39.

The storage package is same as in #49. For every container, it sends container stats to a thread/goroutine pool and worker threads/goroutines will then write those stats to some backend storage.
Hmm, this seems overly complex and maybe not necessary for all storage drivers (think the in-memory one) WDYT of moving this complexity to the drivers that need/want it?