support optional DB alias for old instance lookup

[2ykwang]

code: https://github.com/jazzband/django-auditlog/blob/v3.2.1/auditlog/receivers.py#L50-L68

proposal: change auditlog to default to using instance._state.db when querying for the old instance

old instances may be queried against the wrong database in a multi-DB setup,
leading to cases where auditlog fails to find the previous state or records incorrect diffs
(for example, in a DB replication environment, replication lag could cause unexpected behavior).