Update the interface of Cursor. (#61)
Update TableCursor for the new Cursor. (#61)
Update EmptyCursor for the new Cursor. (#61)
Update IteratorCursor for the new Cursor. (#61)
Update MapSetCursor for the new Cursor. (#61)
Update ArrayCursor for the new Cursor. (#61)
Make Cursor::read() pure virtual. (#61)
Fix a wrong comment.
Remove unused member variables.