cdodge/update-opaque-key: update user social stats to use the legacy courseId format since that is what the comment service expects

fix broken test, make social_stats more robust based on the caller
Merge pull request #160 from edx-solutions/cdodge/update-opaque-key

cdodge/update-opaque-key: update user social stats to use the legacy cou...
cdodge/optimized-save-child-postition: create an optimized version of save_child_position which minimized the number of round trips to MongoDB