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
Merge "Move resource usage sync functions to db backend"