Finally getting around to reviewing the mining interface, and sadly its missing some critical features that a new mining protocol should have. Specifically, one of the key goals of replacing getblocktemplate is that Bitcoin Core should be able to push work to clients at opportune times. This includes the ability to create a new block in between validating a new block and updating the mempool for a full CNB run. Sadly, forcing the client of the interface to explicitly call CNB makes this impossible.