Fix deadlocks when locking job models

[dfaller]

We are occasionally getting exceptions in the message handlers where the database deadlocks on getting model locks on jobs. Check the message handler logs and Postgres logs for more details.

[JohnPTobe]

The messages where I've seen this occur are failed_jobs, completed_jobs and job_exe_end. Add try/catch statements to detect these deadlocks and put the message back on the queue to try again.