`MigrationMode` was kind of defined in the context of an internal mode
used by `migrateBookmark(...)`.
But now, with `getBestTargetBookId`, it is broken.
This commit fix that and the associated implementation.
Now `UPGRADE_ONLY` will make `getBestTargetBookId` return only newer books.
and `ALLOW_DOWNGRADE` will return older books only if current book is
invalid.