Jobs continue to run after unscheduled and killed

Jobs continue to run even after calling `cron.unschedule()` and `pg_cancel_backed()`.

This system was previously running v1.3 and did not have this problem. Recently v1.4 was installed and started noticing this problem.

There is an additional problem that started at the same time for which I will make a separate issue after I'm able to accurately reproduce and document it:

It seems that jobs next in queue will not start until all current jobs are done. Not just jobs with the same `jobname`, but any job regardless of `jobname`. For example, `job_a` is scheduled to run every minute and normally takes 10 seconds to complete. And `job_b` runs every 15 minutes and sometimes takes less, sometimes more, time than that. But `job_a` will not start until `job_b` has completed.

```
\dx pg_cron
List of installed extensions
┌─────────┬─────────┬────────┬──────────────────────────────┐
│ Name │ Version │ Schema │ Description │
├─────────┼─────────┼────────┼──────────────────────────────┤
│ pg_cron │ 1.4 │ ian │ Job scheduler for PostgreSQL │
└─────────┴─────────┴────────┴──────────────────────────────┘
(1 row)

select now();
┌───────────────────────────────┐
│ now │
├───────────────────────────────┤
│ 2022-01-27 09:38:40.540122+00 │
└───────────────────────────────┘

select * from cron.job where jobid=2;
┌───────┬──────────┬─────────┬──────────┬──────────┬──────────┬──────────┬────────┬─────────┐
│ jobid │ schedule │ command │ nodename │ nodeport │ database │ username │ active │ jobname │
├───────┼──────────┼─────────┼──────────┼──────────┼──────────┼──────────┼────────┼─────────┤
└───────┴──────────┴─────────┴──────────┴──────────┴──────────┴──────────┴────────┴─────────┘
(0 rows)

select * from cron.job_run_details where jobid=2 and status='running';
┌───────┬─────────┬─────────┬──────────┬──────────┬─────────────────────────────────────────────────────────────┬─────────┬────────────────┬──────────────────────────────┬──────────┐
│ jobid │ runid │ job_pid │ database │ username │ command │ status │ return_message │ start_time │ end_time │
├───────┼─────────┼─────────┼──────────┼──────────┼─────────────────────────────────────────────────────────────┼─────────┼────────────────┼──────────────────────────────┼──────────┤
│ 2 │ 1475063 │ 31591 │ ..... │ app │ call public.triggers_queue_ ...redacted... │ running │ (null) │ 2022-01-27 09:35:50.58056+00 │ (null) │
└───────┴─────────┴─────────┴──────────┴──────────┴─────────────────────────────────────────────────────────────┴─────────┴────────────────┴──────────────────────────────┴──────────┘
(1 row)

select cron.unschedule(2);
ERROR: could not find valid entry for job 2

select jobid, command, pg_cancel_backend(job_pid) from cron.job_run_details where status='running' and jobid=2;
┌───────┬─────────────────────────────────────────────────────────────┬───────────────────┐
│ jobid │ command │ pg_cancel_backend │
├───────┼─────────────────────────────────────────────────────────────┼───────────────────┤
│ 2 │ call public.triggers_queue_ ...redacted... │ t │
└───────┴─────────────────────────────────────────────────────────────┴───────────────────┘
(1 row)

select * from cron.job_run_details where jobid=2 and status='running';
┌───────┬───────┬─────────┬──────────┬──────────┬─────────┬────────┬────────────────┬────────────┬──────────┐
│ jobid │ runid │ job_pid │ database │ username │ command │ status │ return_message │ start_time │ end_time │
├───────┼───────┼─────────┼──────────┼──────────┼─────────┼────────┼────────────────┼────────────┼──────────┤
└───────┴───────┴─────────┴──────────┴──────────┴─────────┴────────┴────────────────┴────────────┴──────────┘
(0 rows)

select * from cron.job_run_details where jobid=2 and status='running';
┌───────┬─────────┬─────────┬──────────┬──────────┬─────────────────────────────────────────────────────────────┬─────────┬────────────────┬───────────────────────────────┬──────────┐
│ jobid │ runid │ job_pid │ database │ username │ command │ status │ return_message │ start_time │ end_time │
├───────┼─────────┼─────────┼──────────┼──────────┼─────────────────────────────────────────────────────────────┼─────────┼────────────────┼───────────────────────────────┼──────────┤
│ 2 │ 1475073 │ 2430 │ ..... │ app │ call public.triggers_queue_ ...redacted... │ running │ (null) │ 2022-01-27 09:41:46.403165+00 │ (null) │
└───────┴─────────┴─────────┴──────────┴──────────┴─────────────────────────────────────────────────────────────┴─────────┴────────────────┴───────────────────────────────┴──────────┘
(1 row)
```