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

Merge into master from pull request #1636: Open source stuffff (https://github.com/bigswitch/bigswitchcontroller/pull/1636)

Change getFeaturesReply method to 'query' so it's clear this is an external query, and avoids calling it when serializing to JSON.

* Must not call clearAllFlowMods() on slave -> master transition.
* Hack to solve immediate perf problem on switch handshake: only call
  readSwitchPortStateFromStorage() on role change. Switch handshake
  is now fast again but role change is slow (which is has been before)

BSC-2695
BSC-2727

Bug fix

Changes to REST API for links.  http://<controller>:8080/wm/topology/links/json would now return only direct and tunnel links (LinkType.DIRECT_LINK or LinkType.TUNNEL).  To get external links, use http://<controller>:8080/wm/topology/external-links/json.

Add isTunnelPort() to ILinkDiscoveryService.  A tunnel port is not an attachment point port even if no links are found through it.

Fix mismatch between Javadoc comment and method argument

Updates

BDDP packets are now transmitted with BSN Eth-type. Minor refactoring of the code inclues: (1) Removal of BDDP class; (2) Using LLDP encapsulated inside BSN header for BDDP functionality.