loopd: loop in from external address

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.

loopdb/loopin.go

@@ -19,6 +19,10 @@ type LoopInContract struct {
 	// LoopInChannel is the channel to charge. If zero, any channel may
 	// be used.
 	LoopInChannel *uint64
+
+	// ExternalHtlc specifies whether the htlc is published by an external
+	// source.
+	ExternalHtlc bool
 }
 
 // LoopIn is a combination of the contract and the updates.
@@ -100,6 +104,10 @@ func serializeLoopInContract(swap *LoopInContract) (
 		return nil, err
 	}
 
+	if err := binary.Write(&b, byteOrder, swap.ExternalHtlc); err != nil {
+		return nil, err
+	}
+
 	return b.Bytes(), nil
 }
 
@@ -164,5 +172,9 @@ func deserializeLoopInContract(value []byte) (*LoopInContract, error) {
 		contract.LoopInChannel = &loopInChannel
 	}
 
+	if err := binary.Read(r, byteOrder, &contract.ExternalHtlc); err != nil {
+		return nil, err
+	}
+
 	return &contract, nil
 }

loopdb/store_test.go

@@ -207,6 +207,7 @@ func TestLoopInStore(t *testing.T) {
 		},
 		HtlcConfTarget: 2,
 		LoopInChannel:  &loopInChannel,
+		ExternalHtlc:   true,
 	}
 
 	// checkSwap is a test helper function that'll assert the state of a