Refuse to load a wallet if it requires a rescan lower than the height of
an unvalidated snapshot we're running -- in more general terms, if we
don't have data for the blocks.
This is a backport of [[https://github.com/bitcoin/bitcoin/pull/23997 | core#23997]] and [[https://github.com/bitcoin/bitcoin/pull/26282 | core#26282]]
Depends on D17645