sweepbatcher: unit tests with SQL loopdb as well

This is needed to cover the code of SQLStore with tests.

To achieve compatibility with loopdb (SQLite), the following changes were done:

 - DestAddr is filled to avoid crash in SQL layer
 - Preimage is filled to avoid uniqueness checks by the DB
 - the code working with batch IDs was changed to work correctly
   when batch_id starts with 1 instead of 0
 - SQL swap store has to have a swap with swap_hash from sweep
   to satisfy foreign key constraint of the DB

Author: starius