R1:b62420e6e4e0

Separate reading object values out of didSetObject() in CustomField

Summary:
Ref T3886. Broadly, fields break down into two types right now: fields which store data on the object (like `DifferentialTitleField`) and fields which store data in custom field storage.

The former type generally reads data from the object into local storage prior to editing, then writes it back afterward. Currently, this happens in `didSetObject()`.

However, now that we load and set objects from ApplicationTransactionQuery…