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.
