cdodge/optimized-save-child-postition: create an optimized version of save_child_position which minimized the number of round trips to MongoDB

don't refetch the current child, it's not really needed and it can be a bit expensive
Merge pull request #162 from edx-solutions/cdodge/optimized-save-child-postition

cdodge/optimized-save-child-postition: create an optimized version of sa...