remove implicit invocation of session.add() in MappedObject.__init__

Change-Id: I11ab3fcde8fa94d774d72aca97f7ca241b101707
remove unused SystemObject base class

Change-Id: Iaea72c8b878e09b4db3f9e7cf0338ba63500c7c0
port model classes to SQLAlchemy declarative style

Change-Id: I88a1bd869362b4d60d1357288e39cbe962a4ad22