bugfix: overflow of graphx Edge compare function

Author: Zhen Peng <zhenpeng01@baidu.com>

Closes #769 from zhpengg/bugfix-graphx-edge-compare and squashes the following commits:

8a978ff [Zhen Peng] add ut for graphx Edge.lexicographicOrdering.compare
413c258 [Zhen Peng] there maybe a overflow for two Long's substraction