Update heterogeneous sampling to push functionality into the sampling primitives.

[ChuckHastings]

#4608 provides an initial implementation of heterogeneous sampling that implemented the public API by iterating over the edge types. If there are n edge types, this will result in n iterations over the vertices in the sampling frontier to perform the sampling, each using a different edge mask.

It should be more efficient to push the heterogeneous sampling down into the graph primitives. That way in one pass through the vertices we can randomly sample all required edges, regardless of type.