Retain the original legacy field value to entity in sync

There are a lot of use cases, where the original value of the replaced legacy field needs to be retained, to be available on loaded and entites with synced replacement fields, so the original value can be used programatically, if needed. Examples are:

* consistent taxonomy term sorting (term, name(original))
* admin overviews (where you perhaps want to display the original title of a node, for example)

Let's save these original legacy values to an LEGACYFIELDNAME_original property of the synced entity in title_field_sync_get();