[Graph]: use compressed in-memory vertex index by default.
[Graph]: use compressed in-memory vertex index by default.

It only works for directed graphs.

icoming/FlashX,flashxio/FlashX,flashxio/FlashX,icoming/FlashX,silky/FlashGraph,silky/FlashGraph,zheng-da/FlashX,silky/FlashGraph,zheng-da/FlashX,icoming/FlashX,icoming/FlashGraph,icoming/FlashGraph,flashxio/FlashX,flashxio/FlashX,zheng-da/FlashX,flashxio/FlashX,silky/FlashGraph,silky/FlashGraph,icoming/FlashGraph,zheng-da/FlashX,icoming/FlashGraph,icoming/FlashX,icoming/FlashGraph,zheng-da/FlashX,flashxio/FlashX,icoming/FlashX