Finish DB session cleanup

Remove session parameter from:
 agent_build_get_by_triple()
 bw_usage_update()
They are never called with session parameter

Rename aggregate_get_query() -> _aggregate_get_query()
because it is private method

Enable N309

implement blueprint db-session-cleanup

Change-Id: Idd43d230105cb140d8825a251f03a58ea28cce90
Merge "Finish DB session cleanup"