Don't consider specific user-agents and robots to be online users [feature request]

By default, Tracker writes a session to tracker_sessions.

I've created a simple route as follows:

```php
Route::get('/test/', function () {
return Tracker::onlineUsers()->count();
});
```

```bash
$ curl http://localhost:8000/test/
1

$ curl http://localhost:8000/test/
2

$ curl http://localhost:8000/test/
3

$ curl http://localhost:8000/test/
4
```

This is rather undesired behavior because we create sessions for all kinds of robots which should not to be considered online users.

I think a better solution is to not create sessions for robots. What do you think?

Start with the session-writing path used by Tracker::onlineUsers and the tracker_sessions table, then determine how the request user-agent can be identified there. The work is done when the specified robots and user-agents no longer create sessions or count as online users, while ordinary requests retain the existing behavior.

Written by the indexing model from the issue text.