Graceful shutdown during db migration

[IronGauntlets]

If there is a long db migration taking place there is no way to gracefully Juno. Any force shutdown corrupts the DB.

Juno should be able to gracefully shutdown during the migration and pick up where it left off when Juno is restarted.

[joshklop]

Linked PR allows for cancellation between migrations, but not during a migration. The latter was deliberately left for after regenesis.