lnd: add async updates for [open|close]channel RPC's

This commit modifies the internal workflow for opening or closing a
channel in order to create a path in which RPC clients can receive
updates. Updates are now communicated via channels from the goroutines
spawned by the RPC server to process the request, and the sub-system
within the daemon that actually executes the request.

With this change clients can now receive updates that the request is
pending (final message has been sent to the target client), or that the
request has been completed. Confirmation related updates have not yet
been implemented as that will require some changes to the ChainNotifier
interface.

htlcswitch.go

@@ -8,6 +8,7 @@ import (
 	"time"
 
 	"github.com/lightningnetwork/lnd/channeldb"
+	"github.com/lightningnetwork/lnd/lnrpc"
 	"github.com/lightningnetwork/lnd/lnwire"
 	"github.com/roasbeef/btcd/wire"
 	"github.com/roasbeef/btcutil"
@@ -325,7 +326,6 @@ func (h *htlcSwitch) handleUnregisterLink(req *unregisterLinkMsg) {
 func (h *htlcSwitch) handleCloseLink(req *closeLinkReq) {
 	targetLink, ok := h.chanIndex[*req.chanPoint]
 	if !ok {
-		req.resp <- nil
 		req.err <- fmt.Errorf("channel point %v not found", req.chanPoint)
 		return
 	}
@@ -396,27 +396,20 @@ func (h *htlcSwitch) UnregisterLink(chanInterface [32]byte, chanPoint *wire.OutP
 type closeLinkReq struct {
 	chanPoint *wire.OutPoint
 
-	resp chan *closeLinkResp
-	err  chan error
-}
-
-// closeChanResp is the response to a closeChanReq is simply houses a boolean
-// value indicating if the channel coopertive channel closure was succesful or not.
-type closeLinkResp struct {
-	txid    *wire.ShaHash
-	success 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 *closeLinkResp, chan error) {
-	respChan := make(chan *closeLinkResp, 1)
+func (h *htlcSwitch) CloseLink(chanPoint *wire.OutPoint) (chan *lnrpc.CloseStatusUpdate, chan error) {
+	updateChan := make(chan *lnrpc.CloseStatusUpdate, 1)
 	errChan := make(chan error, 1)
 
-	h.linkControl <- &closeLinkReq{chanPoint, respChan, errChan}
+	h.linkControl <- &closeLinkReq{chanPoint, updateChan, errChan}
 
-	return respChan, errChan
+	return updateChan, errChan
 }
 
 // linkInfoUpdateMsg encapsulates a request for the htlc switch to update the