16be13345 Fix rescanblockchain rpc to property report progress (Ben Woosley)
Pull request description:
Previously it assumed tip in all cases. This also extracts a RescanVerificationProgress helper object whose role is to manage reporting, in order to simplify ScanForWalletTransactions - more lines in total, but much simpler to follow the core logic.
Tree-SHA512: 5ebed0c56fae4ccfe613ff1d7082cb6da5a86635a8993ed3af70b500a4ea43074121aea9219b2f0321fbfeb7efcb964bdc2199297a64ca0fa85d9d07aa637d40
Backport of Core PR13079
https://github.com/bitcoin/bitcoin/pull/13079/