fixed import race in db/alchemy/db_utils.py
fixed import race in db/alchemy/db_utils.py

SessionCache used to work only if module "orm" from sqlalchemy
was already imported somewhere


Former-commit-id: 29ca77c17eb5c364bdcd9223c9d7fa917e6e8d8f [formerly fdd81b6046c04d2a9362ceb316cd34986492f6cb]
Former-commit-id: adc2401ecd31364527883844374ef2e1f37ee842
gem/oq-engine,gem/oq-engine,gem/oq-engine,gem/oq-engine,gem/oq-engine