Warn user if the state is probably corrupted (one of previous migrations was interrupted)

[EdvardD]

Config changes:

• Added a flag 'fail_if_migration_is_in_progress' (default to false) to fail node if previously migration was interrupted.

State changes:

• Added a key 'migration_is_in_progress' to the column BlockMisc. Normally the flag doesn't exist or equals false. Equals true iff the last migration was interrupted in the middle. If the flag is true the error message will be shown on the node start. Depending on the flag 'fail_if_migration_is_in_progress' node start will fail.
• Added a key 'store_is_probably_broken'. Normally the flag doesn't exist. Equals true iff previously the migration was done having another interrupted migration. The flag is permanent and is used to show an error message so the user is aware that the state can be corrupted.

[nikurt]

I've just made #6282 which doesn't write anything to DB, but makes a persistent checkpoint to disk. This way we can not only detect if DB is probably corrupted, but can also recover without re-downloading a snapshot.

[abacabadabacaba]

Why don't use transactions to make migrations atomic?

[EdvardD]

Why don't use transactions to make migrations atomic?

It seems a single migration can be too big (the whole storage update) and putting it into a single transaction can lead to issues. That's my guess seeing the implementation.

Anyways @nikurt 's change uses checkpoints to guarantee migrations being transactional so I guess this change is not needed anymore.

[stale]

This PR has been automatically marked as stale because it has not had recent activity in the 2 weeks.
It will be closed in 3 days if no further activity occurs.
Thank you for your contributions.

[mm-near]

let's abandon this PR for now - in favor of @nikurt approach with checkpoints.