Creates a reference to an existing object in the database.
NOTE: The reference may be dangling if the object is not present in the database.
NOTE: Difference from Ref.fromDXN
Ref.fromDXN(dxn) returns an unhydrated reference. The .load and .target APIs will not work.
db.makeRef(dxn) is preferable in cases with access to the database.
ECHO Database interface.