Project Zipper Part 1 - Framework and RGWRadosStore

Project Zipper Part 1 - Framework and RGWRadosStore

This is the first part of Project Zipper, the Store Abstraction Layer.
It introduces the basic framework, and wraps RGWRados in RGWRadosStore.
The goal over the next few weeks is to do the same for user, bucket, and
object.  This will make most of the remaining users of RGWRados wrapped
in SAL classes, allowing it to be completely absorbed into the private
RGWRadosStore.  This will also expose all the APIs that need to be
pusheg up to higher layers in the SAL.

Signed-off-by: Daniel Gryniewicz <dang@redhat.com>