- Uses an instance of the Carrier so it can benefit from issued_receipts and avoid resending multiple copies of the same triggered appointment
- Defines last_known_block to properly load data from db
- Uses task_done from Queue to properly signal task completion when boostraping from db
- Creates a checked_txs dict in get_completed_trackers to avoid querying bitcoind for the same transaction over and over
- Redefines completed_trackers as dict instead of tuple