To allow users to specify differing confirmation targets, we store the
swap conf target per-swap. This makes us restart safe, so we do not
forget confirmation values for swaps that are in flight when we restart.
Once we have revealed our preimage to the world with a sweep attempt,
we can safely push our preimage to the server to speed up on chain
claim.
Rather than rely on the server, we use the state of our invoice in lnd
to determine whether we should continue trying to push the preimage to
the server.
