[worker] Update all send_multipart call and remove str

In Python 3, network is using bytes.

beat/core/worker.py

@@ -87,7 +87,7 @@ class WorkerController(object):
     def destroy(self):
         for worker in self.workers:
             self.socket.send_multipart([
-              str(worker),
+              worker,
               WorkerController.SCHEDULER_SHUTDOWN,
             ])
 
@@ -106,24 +106,24 @@ class WorkerController(object):
 
     def execute(self, worker, job_id, configuration):
         self.socket.send_multipart([
-          str(worker),
+          worker,
           WorkerController.EXECUTE,
-          str(job_id),
-          simplejson.dumps(configuration)
+          b'{}'.format(job_id),
+          simplejson.dumps(configuration).encode('utf-8')
         ])
 
 
     def cancel(self, worker, job_id):
         self.socket.send_multipart([
-          str(worker),
+          worker,
           WorkerController.CANCEL,
-          str(job_id)
+          b'{}'.format(job_id),
         ])
 
 
     def ack(self, worker):
         self.socket.send_multipart([
-          str(worker),
+          worker,
           WorkerController.ACK
         ])