Comment 3 for bug 1068022

Something we could consider to "bullet-proof" oq-engine:

- When a job runs, query the postgres server for the max number of available connections (see http://stackoverflow.com/questions/8288823/query-a-parameter-postgresql-conf-setting-like-max-connections)
- Use the Celery Inspect API to check the total number of worker processes
- If the number of worker procs * 2 > max number of DB connections, abort the job display an error.