* Move Proof and Difficulty types to pow module. Difficulty scaling calculation.
* Diffculty scaling and size shift for 2nd PoW
* Backport e7eb26ee
* Test compilation fixes
* Scaling only need to go one way, as @tromp pointed out
* Remove now unnecessary txhashset write lock
* Ring buffer of hashes the chain has already processed
* Specifically report too old blocks as peer should be banned
* Move sync check for block relay, clean TODO
* No use processing transactions when syncing
* Ignore blocks older than horizon in pruning nodes
