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.
SwapHash used to be a key in the sweeps table. Now the key is outpoint which
replaces columns outpoint_txid and outpoint_index. In-memory structures and unit
tests were also updated to use outpoint as key. Outpoint is truly unique.
This data is not used by Batcher since commit
"sweepbatcher: load swap from loopdb, not own store".
Now sweepbatcher.Store depends only on tables sweeps and sweep_batches
and does not depend on swaps, loopout_swaps and htlc_keys, making it
easier to reuse.
