Create mixin class for common DB fields

Creates a new DBObjectMixin class for DB-backed objects to use.  This
allows us to create non-DB-backed objects without having the common DB
fields automagically added to them.

Related to blueprint unified-object-model

Change-Id: Iaf2d8500505e9acdbffffb1d4bd3db0870ae82a3
Merge "Create mixin class for common DB fields"