routing: launch fetchFundingTx in goroutine so router can exit

This commit introduces a wrapper function fetchFundingTxWrapper
which calls fetchFundingTx in a goroutine. This is to avoid an issue
with pruned nodes where the router is attempting to stop, but the
prunedBlockDispatcher is waiting to connect to peers that can serve
the block. This can cause the shutdown process to hang until we
connect to a peer that can send us the block.

routing/router.go

@@ -1590,7 +1590,7 @@ func (r *ChannelRouter) processUpdate(msg interface{},
 		// to obtain the full funding outpoint that's encoded within
 		// the channel ID.
 		channelID := lnwire.NewShortChanIDFromInt(msg.ChannelID)
-		fundingTx, err := r.fetchFundingTx(&channelID)
+		fundingTx, err := r.fetchFundingTxWrapper(&channelID)
 		if err != nil {
 			// In order to ensure we don't erroneously mark a
 			// channel as a zombie due to an RPC failure, we'll
@@ -1800,6 +1800,36 @@ func (r *ChannelRouter) processUpdate(msg interface{},
 	return nil
 }
 
+// fetchFundingTxWrapper is a wrapper around fetchFundingTx, except that it
+// will exit if the router has stopped.
+func (r *ChannelRouter) fetchFundingTxWrapper(chanID *lnwire.ShortChannelID) (
+	*wire.MsgTx, error) {
+
+	txChan := make(chan *wire.MsgTx, 1)
+	errChan := make(chan error, 1)
+
+	go func() {
+		tx, err := r.fetchFundingTx(chanID)
+		if err != nil {
+			errChan <- err
+			return
+		}
+
+		txChan <- tx
+	}()
+
+	select {
+	case tx := <-txChan:
+		return tx, nil
+
+	case err := <-errChan:
+		return nil, err
+
+	case <-r.quit:
+		return nil, ErrRouterShuttingDown
+	}
+}
+
 // fetchFundingTx returns the funding transaction identified by the passed
 // short channel ID.
 //