Updates Dijkstra's shortest path algorithm. (1) NodeDist will return the node with the smallest weight in the priority queue; (2) The algorithm will replace an existing entry in the priority queue.

This change is mostly for streamlining and clarifying IDeviceServiceAPI.
* renamed findDestDevice to findClassDevice
* pass an entityClass to findClassDevice instead of the
  "source"/"reference" device.

Ok. Let's put the code back in since bigtest is updated now
BSC-2784

This reverts commit 92bfa7a1d71d930f530b8a9cac39c3e6a14a6479.

Cosmetic changes.  Rename varialbe name topology to linkDiscovery, and getTopology to getLinkDiscoveryManager in LinkDiscoveryManagerTest.java.

This reverts commit b3cdf7ae81499f1af2274009633f50c971c4d4d5.

floodlight/OFStatisticsMessageBase: add convenience method that returns the first stats-req/reply message dcontained in the body