Add new all-pairs similarity algorithm

[ChuckHastings]

Added a new entry point for similarity functionality that combines the functionality of k_hop_nbrs and similarity.

This entry point allows us to compute similarity for all pairs of vertices in the graph in a single call. We also add the optional parameter topk which, if specified, will only return the vertices that have the highest scores. If topk is specified on an all pairs call, we compute the scores for pairs in batches and extract the topk as we go along to keep the memory footprint low.

This PR also updates a FIXME in the C++ similarity test. The C++ similarity test had been written before we had a k_hop_nbrs call, so there was some inefficient test code to compute that. Now that we have a k_hop_nbrs call, the test code was refactored to use that call.

Supersedes PR #4134