Make python value for LazyReferenceFields be a DBRef

Previously, when reading a LazyReferenceField from the DB, it was stored
internally in the parent document's _data field as an ObjectId. However, this
meant that equality tests using an enclosing EmbeddedDocument would not return
True when the EmbeddedDocument being compared to contained a DBRef or Document
in _data. Enclosing Documents were largely unaffected because they look at the
primary key for equality (which EmbeddedDocuments lack).

This makes the internal Python representation of a LazyReferenceField (before
the LazyReference itself has been constructed) a DBRef, using code identical to
ReferenceField.

tests/document/test_instance.py

@@ -3321,7 +3321,7 @@ class TestInstance(MongoDBTestCase):
 
     def test_embedded_document_equality_with_lazy_ref(self):
         class Job(EmbeddedDocument):
-            boss = LazyReferenceField('Person')
+            boss = LazyReferenceField("Person")
 
         class Person(Document):
             job = EmbeddedDocumentField(Job)