Add an implementation of our swap interface which can be used for
loop in, and fee estimation. For fee estimation, we always want to
calculate worst case loop in fees, so that autoloop never goes over
its budget. However, for loop in we can't estimate how much a
timeout would cost, because we must sweep the output (can't set
a limit like loop out), and fee estimation in a few hundred blocks
(when we'd sweep the timeout) is totally unreliable.
Instead, we use a high fee rate as our best-effort fee rate for
the future. We can also be confident that that loop in swaps will
succeed, since once the htlc is locked in, all that is required
is for the server to sweep.
