consensus: Fallback to a finalized fork even when this has a lower priority

[fed-franz]

Summary

Implements branch selection and sync logics from DIP proposal #11

TL;DR

After implementing the RF, we need to resolve the case in which a higher iteration block gets finalized but some nodes are still busy with lower iteration blocks. Also nodes should be able to choose between ongoing branches. If seeing two finalized branches, the Provisioner should choose the one with lower iteration.

[autholykos]

This issue needs to be reviewed at the light of what has been done already in terms of patches and the stalled node status