Bug in "MQTT+s3" module: mismatch topics subscribed by clients & failed to train by partial clients per round

Hi, I have found some bugs in `MQTT+s3` module whenI tried to training by partial clients per round.
## **Bug 1: Object of type int64 is not JSON serializable**
```
Traceback (most recent call last):

File "/home/fedml/FedML/python/examples/cross_silo/mqtt_s3_fedavg_mnist_lr_example/one_line/../../../../fedml/core/distributed/communication/mqtt_s3/mqtt_s3_multi_clients_comm_manager.py", line 223, in _on_message_impl
self._notify(payload_obj)
File "/home/fedml/FedML/python/examples/cross_silo/mqtt_s3_fedavg_mnist_lr_example/one_line/../../../../fedml/core/distributed/communication/mqtt_s3/mqtt_s3_multi_clients_comm_manager.py", line 185, in _notify
observer.receive_message(msg_type, msg_params)
File "/home/fedml/FedML/python/examples/cross_silo/mqtt_s3_fedavg_mnist_lr_example/one_line/../../../../fedml/core/distributed/server/server_manager.py", line 111, in receive_message
handler_callback_func(msg_params)
File "/home/fedml/FedML/python/examples/cross_silo/mqtt_s3_fedavg_mnist_lr_example/one_line/../../../../fedml/cross_silo/horizontal/fedml_server_manager.py", line 130, in handle_message_client_status_update
self.send_init_msg()
File "/home/fedml/FedML/python/examples/cross_silo/mqtt_s3_fedavg_mnist_lr_example/one_line/../../../../fedml/cross_silo/horizontal/fedml_server_manager.py", line 70, in send_init_msg
self.send_message_init_config(
File "/home/fedml/FedML/python/examples/cross_silo/mqtt_s3_fedavg_mnist_lr_example/one_line/../../../../fedml/cross_silo/horizontal/fedml_server_manager.py", line 225, in send_message_init_config
self.send_message(message)
File "/home/fedml/FedML/python/examples/cross_silo/mqtt_s3_fedavg_mnist_lr_example/one_line/../../../../fedml/core/distributed/server/server_manager.py", line 114, in send_message
self.com_manager.send_message(message)
File "/home/fedml/FedML/python/examples/cross_silo/mqtt_s3_fedavg_mnist_lr_example/one_line/../../../../fedml/core/distributed/communication/mqtt_s3/mqtt_s3_multi_clients_comm_manager.py", line 284, in send_message
self._client.publish(topic, payload=json.dumps(payload))
File "/home/fedml/anaconda3/envs/fedml/lib/python3.8/json/__init__.py", line 231, in dumps
return _default_encoder.encode(obj)
File "/home/fedml/anaconda3/envs/fedml/lib/python3.8/json/encoder.py", line 199, in encode
chunks = self.iterencode(o, _one_shot=True)
File "/home/fedml/anaconda3/envs/fedml/lib/python3.8/json/encoder.py", line 257, in iterencode
return _iterencode(o, 0)
File "/home/fedml/anaconda3/envs/fedml/lib/python3.8/json/encoder.py", line 179, in default
raise TypeError(f'Object of type {o.__class__.__name__} '
```
The first bug is caused by `aggregator.client_selection()` which return a `numpy.ndarray` indices. However, the `json.dumps()` cannot process `numpy.int64` data.

## **Bug 2: Subscribing mismatch topics**
When I try to train `cnn+mnist` by partial clients per round _(I set `client_num_per_round: 2` before launching 4 clients/ranks for training)_, the sever will be stucked after publishing init messages to the chosen clients.

So, I check the code in `_on_connect_impl` function, and I find that [all clients subscribe `fedml_0_0_1`](https://github.com/FedML-AI/FedML/blob/415674286a168dcb23cfee0b9bb0dbd7a5062628/python/fedml/core/distributed/communication/mqtt_s3/mqtt_s3_multi_clients_comm_manager.py#L140) rather than thier topics `fedml_0_0_{client_id}`. I have no idea why all clients subscribe the `fedml_0_0_1`.

Back to my issue, the server has chosen Client 3 and Client 4 participating the first training round, and sent messages to topic "fedml_0_0_3" and "fedml_0_0_4" respectively. But, they obviously cannot recieve any message as they only subscribes an irrelevant topic "fedml_0_0_1". So, the chosen clients do nothing and the serve gets stuck.

```
############ Server ############
[FedML-Server(0) @device-id-0] [Thu, 26 May 2022 11:54:51] [INFO] [mqtt_s3_multi_clients_comm_manager.py:249:send_message] mqtt_s3.send_message: msg topic = fedml_0_0_3
[FedML-Server(0) @device-id-0] [Thu, 26 May 2022 11:54:51] [INFO] [mqtt_s3_multi_clients_comm_manager.py:256:send_message] mqtt_s3.send_message: S3+MQTT msg sent, s3 message key = fedml_0_0_3_server
[FedML-Server(0) @device-id-0] [Thu, 26 May 2022 11:54:51] [INFO] [mqtt_s3_multi_clients_comm_manager.py:268:send_message] mqtt_s3.send_message: to python client.
{
"msg_type": 1,
"sender": 0, "receiver": 3,
"model_params": "fedml_0_0_3_server",
"client_idx": "993",
"client_os": "PythonClient",
"model_params_url": "https://s3.ai-team.dev/fedml/fedml_0_0_3_server?AWSAccessKeyId="
}

[FedML-Server(0) @device-id-0] [Thu, 26 May 2022 11:54:51] [INFO] [mqtt_s3_multi_clients_comm_manager.py:249:send_message] mqtt_s3.send_message: msg topic = fedml_0_0_4
[FedML-Server(0) @device-id-0] [Thu, 26 May 2022 11:54:51] [INFO] [mqtt_s3_multi_clients_comm_manager.py:256:send_message] mqtt_s3.send_message: S3+MQTT msg sent, s3 message key = fedml_0_0_4_server
[FedML-Server(0) @device-id-0] [Thu, 26 May 2022 11:54:51] [INFO] [mqtt_s3_multi_clients_comm_manager.py:268:send_message] mqtt_s3.send_message: to python client.
{
"msg_type": 1,
"sender": 0,
"receiver": 4,
"model_params": "fedml_0_0_4_server",
"client_idx": "859",
"client_os": "PythonClient",
"model_params_url": "https://s3.ai-team.dev/fedml/fedml_0_0_4_server?AWSAccessKeyId="
}

############ Client 3 ############
[FedML-Client(3) @device-id-1] [Thu, 26 May 2022 11:53:03] [INFO] [mqtt_s3_multi_clients_comm_manager.py:243:send_message] mqtt_s3.send_message: starting...
[FedML-Client(3) @device-id-1] [Thu, 26 May 2022 11:53:03] [INFO] [mqtt_s3_multi_clients_comm_manager.py:327:send_message] mqtt_s3.send_message: MQTT msg sent
[FedML-Client(3) @device-id-1] [Thu, 26 May 2022 11:53:03] [INFO] [mqtt_s3_multi_clients_comm_manager.py:144:_on_connect_impl] mqtt_s3.on_connect: client subscribes real_topic = fedml_0_0_1, mid = 1, result = 0
[FedML-Client(3) @device-id-1] [Thu, 26 May 2022 11:53:03] [INFO] [mqtt_s3_multi_clients_comm_manager.py:164:_on_subscribe] mqtt_s3.onSubscribe: mid = 1

############ Client 4 ############
[FedML-Client(4) @device-id-1] [Thu, 26 May 2022 11:54:47] [INFO] [mqtt_s3_multi_clients_comm_manager.py:243:send_message] mqtt_s3.send_message: starting...
[FedML-Client(4) @device-id-1] [Thu, 26 May 2022 11:54:47] [INFO] [mqtt_s3_multi_clients_comm_manager.py:327:send_message] mqtt_s3.send_message: MQTT msg sent
[FedML-Client(4) @device-id-1] [Thu, 26 May 2022 11:54:47] [INFO] [mqtt_s3_multi_clients_comm_manager.py:144:_on_connect_impl] mqtt_s3.on_connect: client subscribes real_topic = fedml_0_0_1, mid = 1, result = 0
[FedML-Client(4) @device-id-1] [Thu, 26 May 2022 11:54:47] [INFO] [mqtt_s3_multi_clients_comm_manager.py:164:_on_subscribe] mqtt_s3.onSubscribe: mid = 1
```
Furthermore, although "all clients/ranks participating" can run without any exceptions, I thought the above problem still affect model training. Because all clients recieve the same message and use the same `client_idx` sub-dataset. In other words, all clients only train on the same subset of dataset which is from one data holder. The rest of chosen sub-dataset from different data holder are not used.

## **Bug 3: Lack of communication/sync machinism on finishing phase**
Accroding to [code](https://github.com/FedML-AI/FedML/blob/415674286a168dcb23cfee0b9bb0dbd7a5062628/python/fedml/cross_silo/horizontal/fedml_server_manager.py#L207), the server immediately calls `self.finish()` after `self.send_message_sync_model_to_client` without any check or notification in the final round. It's fine for all clients/ranks participating, as their `self.round_idx` is synchronous. So, the server and clients can disconnect and stop correctly.

However, for partial clients participating, it leads to two errors: (1) the server stops while the chosen clients are still processing its message ; (2) the clients cannot exit correctly, because their `self.round_idx` is asynchronous and the server does not notify them "it's time to stop".

I suggest that server and clients should have a communication/sync machinism on finishing phase.