Previously, when handling a sweep we assumed that if a sweep status
was completed, the parent batch was also finished. However, since the
batch confirmation status depends on three on-chain confirmations, it
is possible that a spend notifier was started for a sweep of an active
batch. The notifier would fetch the parent batch from the database, but
because we incorrectly assumed that the parent was confirmed (when it
was not), the DB call would fail with a 'no rows returned' error.
This failure would cause the sweep to fail and the sweep batcher to
stop, resulting in a permanent failure state.
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.
