Split backends off swift/common/db.py

The main purpose of this patch is to lay the groundwork for allowing
the container and account servers to optionally use pluggable backend
implementations. The backend.py files will eventually be the module
where the backend APIs are defined via docstrings of this reference
implementation. The swift/common/db.py module will remain an internal
module used by the reference implementation.

We have a raft of changes to docstrings staged for later, but this
patch takes care to relocate ContainerBroker and AccountBroker into
their new home intact.

Change-Id: Ibab5c7605860ab768c8aa5a3161a705705689b04
Merge "Split backends off swift/common/db.py"