GIRAPH-682: LongNullHashSetEdges performance issue
GIRAPH-667: all workers suspended at 'saveVertices' when use out of core
Merge branch 'trunk' of https://git-wip-us.apache.org/repos/asf/giraph into trunk
GIRAPH-667
GIRAPH-468: Make Vertex an Interface (nitay)
Merge remote-tracking branch 'origin/trunk' into graphviz