wallet: avoid rescans if under the snapshot

Summary:
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 core#23997 and core#26282

Depends on D17645

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D17646