Figure out a better strategy to avoid concurrent Onionoo executions

The current strategy for the hourly cronjob to avoid concurrent executions is to write a lock file at startup and delete it upon termination. And if there's already such a file at startup the cronjob doesn't start.

This strategy works fine if there's a live process not succeeding on time. It fails pretty badly if a process died, because subsequent runs won't start without human intervention.