Debug session should be resilient to brief web-socket disconnects

### Rationale
Long living tcp connections, especially over internet are expected to experience disconnects Theia should be resilient to such disconnects also during debug session.

### Suggested behavior

If the disconnect period is shorter than a predefined threshold the debug session resumes after the reconnect. After the timeout threshold, the behavior should be as today for node.js - session reset.

### Reproduction Steps

- Start node debug session
- Simulate web-socket disconnect

To simulate web-socket disconnects, one can use this code for reverse proxy, and access theia via port 3001

```
var express = require('express');
var app = express();
var httpProxy = require('http-proxy');
var proxy = httpProxy.createProxyServer({ target: 'http://localhost:3000', ws: true });

var server = require('http').createServer(app);

// proxy HTTP GET / POST
app.get('/*', function(req, res) {
console.log("proxying GET request", req.url);
proxy.web(req, res, {});
});
app.post('/*/*', function(req, res) {
console.log("proxying POST request", req.url);
proxy.web(req, res, {});
});

// Proxy websockets
server.on('upgrade', function (req, socket, head) {
console.log("proxying upgrade request", req.url);
proxy.ws(req, socket, head);
});

server.listen(3001);
```

### OS and Theia version:
OS: Debian GNU/Linux
Thiea: 0.4.0