[BSC-3431] Introduce 'direction' as a field in LinkWithType.  The REST API will now return links with direction field that will indicate if the link bidirectional or unidirectional.

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.

Update to link discovery manager.  LLDPs are sent every one second; with a three second timeout on ports where links were already found.  On ports where links are not present, LLDPs are sent once in every 15 seconds.

Conflicts:

	src/main/java/net/floodlightcontroller/util/TimedCache.java

First checking for ITopology REST API. Links between OpenFlow switches connected to the controller can now be seen through /wm/topology/links. [#23518013]