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: fdd81b6046c04d2a9362ceb316cd34986492f6cb
gem/oq-engine,gem/oq-engine,gem/oq-engine,gem/oq-engine,gem/oq-engine