Allow to apply balance updates from migration

This allows to attach balance update receipts from a migration. Because the migration runs before the first block of the new protocol, we cannot return balance updates directly from migration's prepare_first_block. Instead, we add the balance updates receipts when we're finalizing the first block.

This adds a new storage (Storage.Pending_migration_balance_updates) that can be set during migration and checked when we try to finalize a block to apply balance updates receipts.

implements #138 (closed)