cnct: pre-confirmation anchor sweep

Start anchor sweep attempts immediately after the commitment transaction
has been published. This makes the anchor known to the sweeper and
allows the user to bump the fee on it to get their commitment
transaction confirmed in case the fee committed too is insufficient for
timely confirmation.

contractcourt/chain_arbitrator.go

@@ -231,6 +231,33 @@ type arbChannel struct {
 	c *ChainArbitrator
 }
 
+// NewAnchorResolutions returns the anchor resolutions for currently valid
+// commitment transactions.
+//
+// NOTE: Part of the ArbChannel interface.
+func (a *arbChannel) NewAnchorResolutions() ([]*lnwallet.AnchorResolution,
+	error) {
+
+	// Get a fresh copy of the database state to base the anchor resolutions
+	// on. Unfortunately the channel instance that we have here isn't the
+	// same instance that is used by the link.
+	chanPoint := a.channel.FundingOutpoint
+
+	channel, err := a.c.chanSource.FetchChannel(chanPoint)
+	if err != nil {
+		return nil, err
+	}
+
+	chanMachine, err := lnwallet.NewLightningChannel(
+		a.c.cfg.Signer, channel, nil,
+	)
+	if err != nil {
+		return nil, err
+	}
+
+	return chanMachine.NewAnchorResolutions()
+}
+
 // ForceCloseChan should force close the contract that this attendant is
 // watching over. We'll use this when we decide that we need to go to chain. It
 // should in addition tell the switch to remove the corresponding link, such