Do not execute removed cron workloads

[julik]

We need to check whether a workload was enqueued with a scheduler key, but no longer is in the cron table. If that is the case (we are trying to execute a workload which has a scheduler key, but the scheduler does not know about that key) it means that the workload has been removed from the cron table and must not run. Moreover: running it can be dangerous because it was likely removed from the table for a reason. Should that be the case, mark the job "finished" and return nil to get to the next poll. If the deployed worker still has the workload in its scheduler table, but a new deploy removed it - this is a race condition, but we are willing to accept it.

Note that we are already "just not enqueueing" that job when the cron table gets loaded - but it is not enough.
If our system can be in a state of partial deployment:

  [  release 1 does have some_job_hourly crontab entry ]
                 [  release 2 no longer does                           ]
                 ^ --- race conditions possible here --^

...and we remove the crontabled workloads during app boot, it does not give us a guarantee that release 1 won't reenqueue them. For example, via the "reinsert next scheduled" feature when the job is executing. This is why this safeguard is needed.

This protects us from a very dangerous failure mode where we would remove an entry from the cron table, deploy the change, and then still have the workload run a day later.

[svanhesteren]

Awesome! Thanks a lot, also for the cleanups.