[Matrix]: improve merging phase of EM sort.
[Matrix]: improve merging phase of EM sort.

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