Make flatMap functions accept TraversableOnce

[johnynek]

TraversableOnce is a superclass of both Iterable and Iterator. Since we only iterate through once, we could accept that and deal with some objects that can't easily restart (such as iterators from disk etc...)

Shouldn't be a source breaking change.

[johnynek]

Also, we should go through and look at internal uses of map/flatMap and see if we can use iterators to reduce in-memory space.

For instance, in replication in blockJoin and skewJoin, we don't need to materialize the replication fields.