Start API refactor to remove separate Tree

We want trees to just be notes. We want to reference notes
by Path instead of object ids. Stop thinking about the
fs implementation and the fuse service with the same terms -
they will be different backends.

trovedb/db.py

@@ -213,6 +213,7 @@ class Sqlite3Trove:
         if object_id is None:
             object_id = uuid.uuid4()
         sid = _sql_id(object_id)
+        assert sid is not None
 
         # Preserve created timestamp on update
         row = self._con.execute(