In this commit, we introduce a delta from the on-chain HTLC's
expiration. Since clients are now able to specify a confirmation target
for the HTLC sweep, it's possible that the sweep doesn't confirm within
the intended target in the event of an increasing demand for block
space. Once the delta from the HTLC's expiration is reached, we'll begin
to use the default sweep confirmation target when sweeping the HTLC,
which should provide a timely confirmation. This is needed because if
the preimage has already been revealed, then we need to make sure we
sweep the HTLC before the server does.
Allow user to specify that htlc will be published by an external source.
In that case no internal htlc tx will be published.
To facilitate sending to the htlc address, the swap initiation response
is extended with that address.
