Track highest block height instead of just head

[mvandeberg]

Brief description

With the PoB Node Comparator, it is possible that the head block is not the highest block. If that happens and one of the blocks higher than head is built on, the block will get stuck in the applicator. This change allows that block to be applied and progress to be made.

Checklist

• [X] I have built this pull request locally
• [X] I have ran the unit tests locally
• [X] I have manually tested this pull request
• [X] I have reviewed my pull request
• [ ] I have added any relevant tests

Demonstration