* core/state/snapshot: introduce snapshot journal version
* core: update the disk layer in an atomic way
* core: persist the disk layer generator periodically
* core/state/snapshot: improve logging
* core/state/snapshot: forcibly ensure the legacy snapshot is matched
* core/state/snapshot: add debug logs
* core, tests: fix tests and special recovery case
* core: polish
* core: add more blockchain tests for snapshot recovery
* core/state: fix comment
* core: add recovery flag for snapshot
* core: add restart after start-after-crash tests
* core/rawdb: fix imports
* core: fix tests
* core: remove log
* core/state/snapshot: fix snapshot
* core: avoid callbacks in SetHead
* core: fix setHead cornercase where the threshold root has state
* core: small docs for the test cases
Co-authored-by: Péter Szilágyi <peterke@gmail.com>
