adding timeout to cron lock file

fixes https://github.com/owncloud/core/issues/3221

This PR leaves the cron code almost untouched to allow backporting - if required/requested.

Overall the cron code needs refactoring to allow unit testing and proper code structure.

@PVince81 @MorrisJobke @Raydiation please review

@karlitschek @craigpg @MTRichards for your information
What is this? Hours? Days? Minutes? In case its minutes, 30 Minutes is probably too short. The cron is often used for the news app and I think its ok to use a 24h timeout. If you choose a too short timeout you basically take away the reason for it to exist in the first place

As for cron jobs that can take longer than 24 hours: It is advised for large installations to use the API + python updater to update it since its way faster and more reliable. The same should be true for any job that is registered as cronjob. As for ownCloud interna that use it, no idea what a good timeout would be