Better debug log messages
More fine grained emtying of object cache (part 1 to fix #317)

This makes sure that an object isn't in the `session._object_cache`
anymore when `post_update` is called. This avoids recursive SQLAlchemy
`before_commit` event triggers.
Don't do explicit commit (part 2 to fix #317)

Session commit is done in `lib.queue.async.__call__` anyway.