Reduce peak memory requirement of WCC (#4990)

If there exists a high degree vertex, run BFS from the highest degree vertex first to possibly find the largest connected component in the graph. BFS uses a visited flag (1 bit) instead of component ID (sizeof(vertex_t)) to store edge end point (src or dst) properties; thus use less memory.

Then, we can create a smaller graph just extracting edges with unvisited endpoints.

Authors:
  - Seunghwa Kang (https://github.com/seunghwak)

Approvers:
  - Chuck Hastings (https://github.com/ChuckHastings)

URL: #4990

Author: seunghwak