Support for new tx handling 

Added:
* `is_completed` and `part` fields in `transactions` table,
* PRIMARY KEY changed from id to (id, part),
* migration `003`

Rest is just a merge from `migrations` branch.

**DO NOT MERGE** it yet.
It's just a fallback so we don't lose bills with machines that don't yet support this, and bill cannot be inserted to db due to unique violation.