Use header provider callback in QRepObjectSync

[jkaflik]

The header situation is a bit mind-bending. They're queued along with each object, but are actually a reference to the same map, and then object sync assumes they're the same within batch? Wonder if there's a way to have one object (per snapshot) that everyone calls and it's always up to date

Originally posted by @ilidemi in #3782 (comment)