Create and use subclass of sqlalchemy Query with soft_delete() method

Create subclass of sqlalchemy.orm.query.Query that has soft_delete() method

Change get_maker() method to create Session with our Query class

The main idea of soft_delete() method is to get good performance with clean
syntax: Let we have already `query` instance.

Clean syntax example:
If we want to make soft delete for each entry that correspond to query:
we should write:
        query.soft_delete()
instead of:
        query.update({
                deleted:True,
                'deleted_at': timeutils.utcnow(),
                'updated_at': literal_column('updated_at')})

Performance examples:
If we make:
        query.soft_delete()
we will make only one request to soft delete corresponding entries.
But if we make:
        for model_ref in query.all():
                model_ref.delete(session=session)
it will produce >= N + 1 queries.

In almost all situation it is better to use query.soft_delete() than
model.delete() method, because in second approach we should make
2 request (get model_ref and then soft delete it).

In future patch model.delete() will be at least renamed to model.soft_delete().
Probably it should be removed at all, to avoid situation in code with N+1
requests for soft deletion.

blueprint db-session-cleanup

Change-Id: I6059982b83648133b6d8becf94ad8980b056c6ff

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