DiskFile and DatabaseBroker as a Public API

From the OpenStack Summit talk given by Pete Zaitcev on the state of the LFS patch, it became clear that the community would like to define an internal API to allow different backend implementations for how the object, container and account servers behave.

We propose the existing DiskFile class (swift/obj/server.py) be defined as the internal backend API for the object server, and the ContainerBroker and AccountBroker classes (swift/common/db.py) be the backend API for the container and account servers respectively. Some work needs to be done to define how replacement classes are specified and if the existing APIs defined by those classes need adjustments.