Back cron's last-run time somewhere permanent

[PeterJCLaw]

Currently if the routemaster cron processing is delayed it will try to catch up when it next runs. This is handled by remembering the last-run time in memory.

However this means that if the cron processing is delayed and the instance is then restarted that the pending jobs will not get run at all.

This means that running routemaster on cloud services (where instances are likely to be killed arbitrarily) can lose cron tasks.