Our preimage push test previously relied on our dropping down to the
default sweep conf target to mock a drop in chain fees. This makes
our test dependent on height, which makes changes to our sweep logic
regarding when we reveal our preimage break this test. In this commit
that logic is replaced with simply locking our mock and updating fees
on the fly.
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.
This addresses an issue where using a sweep confirmation target greater
than the default would result in most cases not revealing the preimage
due to the default confirmation target yielding a higher fee than the
max miner fee backed by the confirmation target provided.
