### Current Behavior
A Node red instance with a single flow consisting of a sin…gle mqtt-in node and associated broker node prevents the NR instance from exiting cleanly.
### Expected Behavior
Calling RED. stop() should cleanly terminate all flows and allow node to exit,
The issue seems to be that the mqtt code (10-mqtt.js) doesn't call client.close() during its disconnect handler.
https://github.com/node-red/node-red/blob/329008bf6d4325e2ed47437c4824f91202b7ca3f/packages/node_modules/%40node-red/nodes/core/network/10-mqtt.js#L831
Changing this to the following fixes the issue
```
if(node.closing) { node.client.end(); return _callback(); }
```
### Steps To Reproduce
The following code reproduced the problem.
```
const http = require('http');
const express = require("express");
const RED = require("node-red");
const fs = require('fs');
const os = require('os');
const path = require('path');
const whyIsNodeRunning = require('why-is-node-running')
const mqttFlow = {
label: 'mqtt',
disabled: false,
info: '',
nodes: [
{
type: 'mqtt in',
name: 'mqtt',
id: 'b021cb01-cbd5-11ec-8a3d-47d2cdb2f658',
topic: 'myTopic',
broker: 'b021f210-cbd5-11ec-8a3d-47d2cdb2f658'
},
{
type: 'mqtt-broker',
name: '',
id: 'b021f210-cbd5-11ec-8a3d-47d2cdb2f658',
keepalive: 60,
port: '1883',
broker: 'localhost'
},
]
}
process.once('SIGINT', function () {
console.log('SIGINT received...');
close();
});
process.once('SIGTERM', function () {
console.log('SIGTERM received...');
close();
});
var server;
const userDir = fs.mkdtempSync(path.join(os.tmpdir(),'test-'));
async function start () {
let app = express();
server = http.createServer(app);
var settings = {
httpAdminRoot:"/red",
httpNodeRoot: "/api",
userDir: userDir,
};
RED.init(server, settings);
app.use(settings.httpAdminRoot,RED.httpAdmin);
app.use(settings.httpNodeRoot,RED.httpNode);
// Start Node Red
await RED.start()
RED.runtime.events.once('flows:started', () => {
RED.runtime.flows.addFlow({flow: mqttFlow});
})
server.listen(2000);
console.log(`Node Red server running on port 2000`)
}
async function close() {
await RED.stop();
await server.close();
fs.rmSync(userDir, {force: true, recursive: true})
setTimeout(() => {whyIsNodeRunning(console)}, 1000)
}
start();
```
Running this from the command line and stopping it with Ctrl-C leads to the node process hanging and "why-IsNodeRunning" shows that the mqtt reconnect timer is still active.
```
# Timeout
node:internal/async_hooks:203
node:internal/async_hooks:474
node:internal/timers:162
node:internal/timers:196
/home/phil/CT/NRBug/node_modules/mqtt/lib/client.js:1123 - that.reconnectTimer = setInterval(function () {
/home/phil/CT/NRBug/node_modules/mqtt/lib/client.js:390 - this._setupReconnect()
/home/phil/CT/NRBug/node_modules/mqtt/lib/client.js:477 - that.emit('close')
```
[nrBug.tar.gz](https://github.com/node-red/node-red/files/8631673/nrBug.tar.gz)
### Example flow
```
[{"id":"075db3f8c5ea6d56","type":"tab","label":"mqtt","disabled":false,"info":""},{"id":"b021cb01-cbd5-11ec-8a3d-47d2cdb2f658","type":"mqtt in","z":"075db3f8c5ea6d56","name":"mqtt","topic":"myTopic","broker":"b021f210-cbd5-11ec-8a3d-47d2cdb2f658","inputs":0,"x":0,"y":0,"wires":[[]]},{"id":"b021f210-cbd5-11ec-8a3d-47d2cdb2f658","type":"mqtt-broker","z":"075db3f8c5ea6d56","name":"","broker":"localhost","port":"1883","keepalive":60}]
```
### Environment
- Node-RED version: 2.2.2
- Node.js version: v16.4.0
- npm version: 7.18.1
- Platform/OS: Ubuntu 20.04.1
- Browser:Version 98.0.4758.102 (Official Build) (64-bit)
