fix inconsistency in zmqrpc

[delulu]

From zmqrpc.py, there're two new added methods send_to_client and recv_from_client for the messaging with client.

locust/locust/rpc/zmqrpc.py

Lines 14 to 27 in 87a9aa1

	def send_to_client(self, msg):
	self.socket.send_multipart([msg.node_id.encode(), msg.serialize()])
	def recv(self):
	data = self.socket.recv()
	msg = Message.unserialize(data)
	return msg
	def recv_from_client(self):
	data = self.socket.recv_multipart()
	addr = data[0]
	msg = Message.unserialize(data[1])
	return addr, msg

In method send_to_client msg.node_id is supposed to be a str object, while in the context of zmqrpc msg.node_id will be assigned with the addr from the return tuple of recv_from_client, which was a bytes object.

This consistency will cause the encoding issue of addr where it's referenced, including the example below

locust/locust/web.py

Lines 126 to 134 in 87a9aa1

	for slave in runners.locust_runner.clients.values():
	slaves.append({"id":slave.id, "state":slave.state, "user_count": slave.user_count})
	report["slaves"] = slaves
	report["state"] = runners.locust_runner.state
	report["user_count"] = runners.locust_runner.user_count
	return jsonify(report)

the slave.id is also assigned with addr and it will be a bytes object which will cause an exception thrown from jsonify: "TypeError: Object of type 'bytes' is not JSON serializable".

[delulu]

@cgoldberg please have a review and merge into master, or locust won't function well on python3 in the coming release 0.10.0