Some work done on mongo sort.
Support pushing sorts down to MongoDB. Required new rule PushSortPastProjectRule, and some bug fixes in how Optiq deals with subsets. If a rel requires a rel sorted by [x] it should also allow one sorted by [x, y]. A few rules were incorrectly assigning the collation of their result.

Make JoinRel.stoppedVariables an immutable set, and final.

Add Util.isDistinct(), and use it in various places.

Add instructions for loading FoodMart data set into MongoDB.
Merge branch 'lucboudreau-master'