Set deadline on client websocket sends.

It appears that the websocket package will occasionally enter a
Codec's Send function and block forever, never erroring (presumably
due to a closed connection).  This change adds a deadline for the send
of two seconds.  If the send cannot complete before the deadline is
reached, the send is aborted and the connection is assumed to be lost.

A buffer should be added here as well, so even waiting max two seconds
for the send to error out won't cause wallet code to block.

sockets.go

@@ -330,6 +330,8 @@ func WSSendRecv(ws *websocket.Conn) {
 		}
 	}()
 
+	const deadline time.Duration = 2 * time.Second
+
 	for {
 		select {
 		case m, ok := <-received:
@@ -348,7 +350,12 @@ func WSSendRecv(ws *websocket.Conn) {
 			}(m)
 
 		case m := <-cc.send:
-			if err := websocket.Message.Send(ws, m); err != nil {
+			err := ws.SetWriteDeadline(time.Now().Add(deadline))
+			if err != nil {
+				return
+			}
+			err = websocket.Message.Send(ws, m)
+			if err != nil {
 				// Frontend disconnected.
 				return
 			}