Add `for_update=False` to `refresh_from_db()`

[Patryk Zawadzki]

I have a project where we deal with many financial operations that rely on transactions and row-level locking a lot to guard against race conditions. Unfortunately it's a common pattern for a function to accept an instance of model A, start an atomic block, fetch a new instance of the same row with select_for_update(), do a change, call save() and then refresh the original instance. It would all be easier if I could just call instance.refresh_from_db(for_update=True) and it would save a me a DB roundtrip to refresh the original instance passed (Django ORM does not have a session manager so the old object will not otherwise reflect the changes).