fixed bug in golang channel handling (json-rpc mode)

* golang channels are meant to be 1:1 channels, so if multiple
  goroutines listen on the same channel for messages, only one will
  receive the message and the others are not, which lead to lost
  messages.

  In order to fix that, we create a dedicated golang channel for every
  websocket connection.

see #451

src/api/api.go

@@ -390,7 +390,7 @@ func (a *Api) SendV2(c *gin.Context) {
 }
 
 func (a *Api) handleSignalReceive(ws *websocket.Conn, number string, stop chan struct{}) {
-	receiveChannel, err := a.signalClient.GetReceiveChannel()
+	receiveChannel, channelUuid, err := a.signalClient.GetReceiveChannel()
 	if err != nil {
 		log.Error("Couldn't get receive channel: ", err.Error())
 		return
@@ -399,6 +399,7 @@ func (a *Api) handleSignalReceive(ws *websocket.Conn, number string, stop chan s
 	for {
 		select {
 		case <-stop:
+			a.signalClient.RemoveReceiveChannel(channelUuid)
 			ws.Close()
 			return
 		case msg := <-receiveChannel: