Move resource usage sync functions to db backend

Move resource usage sync functions to db backend

Resource usage sync functions was declared in cinder/quota.py, and
using db.api public methods. This functions was moved to database
backend implementation, so now sync functions can use private
methods of database backend, and session attribute can be removed
from this public methods.

Blueprint: db-session-cleanup

Change-Id: If5386e3dc1e0d6e3127732aeb5b35bbd96bc93f0