network: convert links & switches to first class C++ SimObjects

This patch converts links and switches from second class simobjects that were
virtually ignored by the networks (both simple and Garnet) to first class
simobjects that directly correspond to c++ ojbects manipulated by the
topology and network classes.  This is especially true for Garnet, where the
links and switches directly correspond to specific C++ objects.

By making this change, many aspects of the Topology class were simplified.