Split cstoken and stoken to be different concepts

The stokens are really just integrity checks for items, and are really
just tied to what revision we expected to have first what we have. So we
will rename stoken to lastRev or something, and have them completely
separate.

A partial revert of e22a49f982046e875d4e1c5007a91353527d7a0f

django_etesync/models.py

@@ -41,13 +41,13 @@ class Collection(models.Model):
     def main_item(self):
         return self.items.get(uid=None)
 
-    @cached_property
+    @property
     def content(self):
         return self.main_item.content
 
     @property
     def stoken(self):
-        return self.main_item.stoken
+        return self.content.uid
 
     @cached_property
     def cstoken(self):
@@ -78,7 +78,7 @@ class CollectionItem(models.Model):
 
     @property
     def stoken(self):
-        return self.content.stoken.uid
+        return self.content.uid
 
 
 def chunk_directory_path(instance, filename):