What changes were proposed in this pull request?

1. New vertex column in Pregel, related to the flag is the vertex active
2. New options to set initial expression for active flag and update expression
3. New option that allows to stop earlier if all the vertices are non active
4. New option that allows to not generate messages from non active vertices
5. Update ShortesPath with GF implementation with new options

How was it tested?

By GF ShortestPaths tests

Why are the changes needed?

As described in issues:
Close #551

cc: @SauronShepherd