Use uuid as mqtt clientId

Use crypto/random to generate unique mqtt client id. The tile38 prefix
makes the connections easily identifiable in the mqtt broker logs.

internal/endpoint/mqtt.go

@@ -3,12 +3,14 @@ package endpoint
 import (
 	"crypto/tls"
 	"crypto/x509"
+	"crypto/rand"
 	"fmt"
 	"io/ioutil"
 	"sync"
 	"time"
 
 	paho "github.com/eclipse/paho.mqtt.golang"
+	"github.com/tidwall/tile38/internal/log"
 )
 
 const (
@@ -83,9 +85,16 @@ func (conn *MQTTConn) Send(msg string) error {
 			}
 			ops = ops.SetTLSConfig(&config)
 		}
-		nano := time.Now().UnixNano()
-		clientID := fmt.Sprintf("tile38_%x", nano) //the id of connected clients should be unique
-		ops = ops.SetClientID(clientID).AddBroker(uri)
+		//generate UUID for the client-id. 
+		b := make([]byte, 16)
+		_, err := rand.Read(b)
+		if err != nil {
+			log.Debugf("Failed to generate guid for the mqtt client. The endpoint will not work")
+			return err;
+		}
+		uuid := fmt.Sprintf("tile38-%x-%x-%x-%x-%x", b[0:4], b[4:6], b[6:8], b[8:10], b[10:])
+
+		ops = ops.SetClientID(uuid).AddBroker(uri)
 		c := paho.NewClient(ops)
 
 		if token := c.Connect(); token.Wait() && token.Error() != nil {