mgmt+core: Moving SegmentPublisher to core/ and removing KeyChain from nfd::AppFace

This change ensures interface uniformity between nfd::AppFace and
ndn::Face and allow now templated version of SegmentPublisher to be used
with either.

Change-Id: I82e29528b13b37bd2ae27ef47db30e8c87c15a38
Refs: #1749
rib: Add RIB dataset publisher

refs: #1662
Change-Id: I97885c07af131a7ea78d074b39df884178f09416
core: Force SegmentPublisher to publish at least one Data packet, which could have empty content

Change-Id: I8338f78ebfa4c2641eaae606b28c1112f06619c9
Refs: #1662
tools: visualize RIB dataset in nfd-status

refs #1749

Change-Id: Ia11b88187635ffa4eda4b829afd6a20120765d58