[Command] Added an helper to lock a command

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #9357 , #3586
| License       | MIT
| Doc PR        | https://github.com/symfony/symfony-docs/pull/3956/files

Code sample:

```php
    /**
     * {@inheritdoc}
     */
    protected function execute(InputInterface $input, OutputInterface $output)
    {
        $lockHelper = new LockHelper('/tmp/acme/hello.lock');
        if (!$lockHelper->lock()) {
            $output->writeln('The command is already running in another process.');

            return 0;
        }

        $output->writeln(sprintf('Hello <comment>%s</comment>!', $input->getArgument('who')));

        for (;;) {
        }

        $output->writeln(sprintf('bye <comment>%s</comment>!', $input->getArgument('who')));

        $lockHelper->unlock();
    }
```

![process-lock](https://f.cloud.github.com/assets/408368/2443205/4f0bf3e8-ae30-11e3-9bd4-78e09e2973ad.png)

> We are using the same file over and over. So we are not going to reach max file issue.

But the file name is passed to the constructor. So, if you have many calls of it each with different files, that might become an issue.