This is a draft of the heartbeat feature. When -enablertt=1, the node will reject blocks that are received too soon after the previous one.
Based on the research from https://ledger.pitt.edu/ojs/ledger/article/download/195/187/1008.
This needs to be split into pieces, and ideally the double computation should be removed for int256 computation. I also left some debug prints under a new experimental category that should eventually get removed.
Depends on D16593 and D16594.