Move resource usage sync functions to db backend

Resource usage sync functions was declared in nova/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, and even some public methods can be removed.

Blueprint: db-session-cleanup

Change-Id: I63eeb18b8ede4b4263ee553d4e70633de463e0c4
Merge "Move resource usage sync functions to db backend"