Add vote-graph nodes for fork blocks

[rphmeier]

Currently, we only add vote-graph nodes for blocks which have been voted upon. Because of this, in the GHOST operation, we sometimes need to find a "merge point" under the following circumstances

• We have a node which fulfills a condition (i.e. supermajority)
• The node's children don't fulfill the condition
• We need to find the highest descendent block of the node (possibly the node block itself) which fulfills the condition.

If we had nodes for fork blocks as well as voted-on blocks, the "merge" point operation (potentially expensive) would become unnecessary. The downside is that the add_vote operation may introduce up to two nodes to the graph, doubling the number of nodes and cost of search operations in the worst case.