In this commit we add a new function SelectDeposits
to the loop-in manager. It coin-selects deposits that
meet an arbitrary swap amount provided by the client.
We have to ensure that the server creates the correct
change outputs for the htlc- and sweepless sweep
transactions.
This commit adds the listening for sweep requests from the server.
On a sweep request the loopin manager will fetch the loop in from the
db, do sanity and safety checks and then sign the psbt for the input
and send it back to the server.
