lnd: properly execute force closures kicked off via RPC

This commit includes some slight refactoring to properly execute force
closures which are initiated by RPC clients.

The CloseLink method within the htlcSwitch has been extended to take an
additional parameter which indicates if the link should be closed
forcefully. If so, then the channelManager which dispatches the request
executes a force closure using the target channel state machine. Once
the closing transaction has been broadcast, the summary is sent to the
utxoNursery so the outputs can be swept once they’re mature.

htlcswitch.go

@@ -63,7 +63,13 @@ type htlcSwitch struct {
 	started  int32 // atomic
 	shutdown int32 // atomic
 
-	chanIndex  map[wire.OutPoint]*link
+	// chanIndex maps a channel's outpoint to a link which contains
+	// additional information about the channel, and additionally houses a
+	// pointer to the peer mangaing the channel.
+	chanIndex map[wire.OutPoint]*link
+
+	// interfaces maps a node's ID to the set of links (active channels) we
+	// currently have open with that peer.
 	interfaces map[wire.ShaHash][]*link
 
 	// TODO(roasbeef): msgs for dynamic link quality
@@ -395,20 +401,29 @@ func (h *htlcSwitch) UnregisterLink(chanInterface [32]byte, chanPoint *wire.OutP
 // closeChanReq represents a request to close a particular channel specified
 // by its outpoint.
 type closeLinkReq struct {
-	chanPoint *wire.OutPoint
+	chanPoint  *wire.OutPoint
+	forceClose bool
 
 	updates chan *lnrpc.CloseStatusUpdate
 	err     chan error
 }
 
 // CloseLink closes an active link targetted by it's channel point. Closing the
-// link initiates a cooperative channel closure.
-// TODO(roabeef): bool flag for timeout/force
-func (h *htlcSwitch) CloseLink(chanPoint *wire.OutPoint) (chan *lnrpc.CloseStatusUpdate, chan error) {
+// link initiates a cooperative channel closure iff forceClose is false. If
+// forceClose is true, then a unilateral channel closure is executed.
+// TODO(roabeef): bool flag for timeout
+func (h *htlcSwitch) CloseLink(chanPoint *wire.OutPoint,
+	forceClose bool) (chan *lnrpc.CloseStatusUpdate, chan error) {
+
 	updateChan := make(chan *lnrpc.CloseStatusUpdate, 1)
 	errChan := make(chan error, 1)
 
-	h.linkControl <- &closeLinkReq{chanPoint, updateChan, errChan}
+	h.linkControl <- &closeLinkReq{
+		chanPoint:  chanPoint,
+		forceClose: forceClose,
+		updates:    updateChan,
+		err:        errChan,
+	}
 
 	return updateChan, errChan
 }