Throttle number of publish messages

[johnhydroware]

Some MQTT brokers have an limit om number of publish messages(AWS IoT). Since resend and other functionality is build into the client, this can't easily be handled by the application code. Therefore I suggest an throttle option. See code below for my suggestion on a parameter to set number of messages per second. I'm not super happy about the implementation. Any suggestions are welcome:

index 83349a5..0671834 100644
--- a/net.go
+++ b/net.go
@@ -165,14 +165,33 @@ func incoming(c *client) {
 func outgoing(c *client) {
        defer c.workers.Done()
        DEBUG.Println(NET, "outgoing started")
+       var ticks <-chan time.Time
+       var throttleTicker *time.Ticker
+
+       obound, oboundP := c.obound, c.oboundP
+
+       if c.options.PublishThrottle > 0 {
+               throttleTicker = time.NewTicker(time.Duration(int64(time.Millisecond) * int64(1000/c.options.PublishThrottle)))
+               defer throttleTicker.Stop()
+               ticks = throttleTicker.C
+       }
 
        for {
                DEBUG.Println(NET, "outgoing waiting for an outbound message")
                select {
+               case <-ticks:
+                       obound = c.obound
+                       oboundP = c.oboundP
+                       ticks = nil
                case <-c.stop:
                        DEBUG.Println(NET, "outgoing stopped")
                        return
-               case pub := <-c.obound:
+               case pub := <-obound:
+                       if c.options.PublishThrottle > 0 {
+                               obound = nil
+                               oboundP = nil
+                               ticks = throttleTicker.C
+                       }
                        msg := pub.p.(*packets.PublishPacket)
 
                        if c.options.WriteTimeout > 0 {
@@ -196,7 +215,12 @@ func outgoing(c *client) {
                                pub.t.flowComplete()
                        }
                        DEBUG.Println(NET, "obound wrote msg, id:", msg.MessageID)
-               case msg := <-c.oboundP:
+               case msg := <-oboundP:
+                       if c.options.PublishThrottle > 0 {
+                               obound = nil
+                               oboundP = nil
+                               ticks = throttleTicker.C
+                       }
                        switch msg.p.(type) {
                        case *packets.SubscribePacket:
                                msg.p.(*packets.SubscribePacket).MessageID = c.getID(msg.t)