Add flock to config

This adds a file lock to the config in hope that this prevents race conditions as reported in https://github.com/owncloud/core/issues/11070

Testplan:

- [ ] Delete config.php and make it read-only => Error is thrown that it is not writeable
- [ ] Installation still works
- [ ] Changing config settings works (i.e. using the SMTP config switches in the administration menu)
- [ ] Your PC didn't blow up
- [ ] Installing the news app and the "Disable AppCode checker" app did not destroy your installation

@bantu Please review. Much appreciated, thanks!
@PVince81 Your feedback would also be very welcome!
I see. It seems that the files_locking app also use some kind of code gymnastics:
https://github.com/owncloud/apps/blob/master/files_locking/lib/lock.php#L131

It seems to create a second lock file, considering that other file access would go through that app as well and can be blocked by that file.

Yeah, then just keep it as is if it works that way.