Update graph primitives to support vertex-tagging in frontier expansion

[seunghwak]

To get early feedback and to create a baseline weakly-connected-component PR dependent on this PR.
This PR requires C++17 and will not compile till #1528 gets resolved.

Tagging vertex IDs with root IDs is necessary in

multi-source BFS, k-hop neighbors, and the newly designed weakly-connected-component algorithm based on multi-root collaborative frontier expansion.

To fully support multi-source BFS & k-hop neighbors, we also need to store/query properties for (vertex, tag) pairs, and this update will be added in a future PR.

[ChuckHastings]

LGTM so far.

[codecov-commenter]

Codecov Report

Merging #1551 (c8eddd1) into branch-0.20 (250f2c8) will decrease coverage by 1.13%.
The diff coverage is 62.32%.

@@               Coverage Diff               @@
##           branch-0.20    #1551      +/-   ##
===============================================
- Coverage        61.04%   59.90%   -1.14%     
===============================================
  Files               73       79       +6     
  Lines             3219     3482     +263     
===============================================
+ Hits              1965     2086     +121     
- Misses            1254     1396     +142     

Impacted Files	Coverage Δ
python/cugraph/__init__.py	100.00% <ø> (ø)
python/cugraph/components/connectivity.py	95.65% <ø> (ø)
python/cugraph/dask/centrality/katz_centrality.py	29.16% <ø> (ø)
python/cugraph/dask/link_analysis/pagerank.py	21.87% <0.00%> (ø)
python/cugraph/dask/traversal/bfs.py	25.00% <0.00%> (-2.59%)	⬇️
python/cugraph/traversal/bfs.py	89.65% <ø> (+2.69%)	⬆️
python/cugraph/traversal/ms_bfs.py	11.11% <ø> (ø)
python/cugraph/traversal/sssp.py	92.85% <ø> (-0.78%)	⬇️
python/cugraph/traversal/traveling_salesperson.py	84.61% <ø> (ø)
python/cugraph/tree/minimum_spanning_tree.py	85.36% <ø> (ø)
... and 45 more

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[BradReesWork]

@gpucibot merge