Added option WithSkippedTxns, which has one historical problematic tx by default.
Sweeps originating from these transactions are omitted when reading from DB.
loopdb: add column sweep_batches.cancelled and replaced DropBatch with
CancelBatch. It is needed, because sweep.batch_id is a foreign key to batch.
Changed StoreMock.InsertSweepBatch not to reuse batch_id. This is needed by
the test, which checks that new batch has fresh ID.
