replayer: factor out legacy objects fixers
This will allow other journal clients to use the fixers, instead of this being solely a replayer feature.
One could also imagine integrating the fixer directly in the journal so that the fixes happen transparently for all the users without having to call the fixer explicitely, and this diff is a first step in this direction.
Test Plan
tox passes the unit tests and mypy is happy
Migrated from D2868 (view on Phabricator)