Update triangle count to internally use edge masking and update Triangle counting & K-hop neighbor tests to include edge masked graph view inputs

[seunghwak]

This PR pulls updates from #4253
Better be reviewed after #4253 is merged.

• Update triangle count to internally use edge masking (this is cheaper than creating a new graph in both memory footprint and compute) to
  • include only the edges with endpoints reachable within two hops (if the list of vertices to compute triangle counts is provided)
  • Exclude self-loops
  • Exclude vertices that do not belong to 2-Core
  • Note that we are still creating a new graph when we are creating a degree ordered directed graph (as this graph is now significantly smaller than the original graph and edge masking has non-negligible overhead and computing triangle counts on the final degree ordered directed graph is computationally intensive).
• Update SG/MG Triangle counting & K-hop neighbors C++ tests to include edge masked graph view inputs

[naimnv]

LGTM

[ChuckHastings]

/merge