Provide scroped_session context manager

[rickmak]

It is observed that various plugin includes the following function for working with DB directly.

I think we can provide the same function. The good thing of we providing the function gives us a way to config the scoped_session that is the best fit for cloud deployment and different transport.

import logging

from contextlib import contextmanager
from skygear.utils.db import _get_engine
from sqlalchemy.orm import sessionmaker

log = logging.getLogger(__name__)
Session = sessionmaker()


@contextmanager
def scoped_session(session=None):
    if session is None:
        session = _create_session()
    try:
        yield session
        session.commit()
    except:
        log.warn('rollback session')
        session.rollback()
        raise
    finally:
        session.close()


# Usage
with scoped_session() as session:
    session.add(user)