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.
Perhaps, ```partial_id```. The problem with ```part``` is that it's a very general term that doesn't indicate that it's a sequence number.