Add new implementation of XPregel
Commit modified files.
Modify old implements of XPregel
Remove test.xpregel
Changed Xpregel APIs
Modify codes due to compilation errors
fix miss of bound check of Bitmap
Change XPregel: use GraphEdge and GraphEdgeBase, remove mOutOffsets, mOutVertex, ...
Merge branch 'feature/xpregel' into feature/bc