make CronJobManager be a context manager. This simplifies it's usage and allows to log all errors into db.
move cron job lock into CronJobManager.run
move cron job lock into CronJobManager.run
use CronJobManage as a context manager at runcrons management command
fix cron job locking backends to suit CronJobManager's needs. Return lock fail messages instead of printing them directly
fix cron job FailedRunsNotificationCronJob to import get_class routine from new place