Consider delaying the detection of modified key values to SaveChanges

[AndriySvyryd]

This brings the handling of PropertySaveBehavior to one place, removes the need for the SavingChanges flag added in #19100 and is needed to allow reparenting of dependents with identifying FKs.

[ajcvickers]

When fixing this, the special check to prevent changing a key added in #22322 can be removed.

Also, consider doing this at the same time as #4073