Limit the started jobs per "dataset namespace"
The env var `MAX_JOBS_PER_DATASET` is rename `MAX_JOBS_PER_NAMESPACE`.

Also: select the next job among the namespaces with the least number of started jobs in order to avoid having all the workers dedicated to the same user if others are waiting. The namespace is the user, the org, or the dataset name for canonical datasets.

**Deployment:** the collection is now called jobs_blue (see blue/green deployment). Once deployed, look at the old "jobs" collection, relaunch the jobs for the waiting or started datasets, then delete the "jobs" collection.
Limit the started jobs per "dataset namespace": The env var `MAX_JOBS_PER_DATASET` is rename `MAX_JOBS_PER_NAMESPACE`.

Also: select the next job among the namespaces with the least number of started jobs in order to avoid having all the workers dedicated to the same user if others are waiting. The namespace is the user, the org, or the dataset name for canonical datasets.

**Deployment:** the collection is now called jobs_blue (see blue/green deployment). Once deployed, look at the old "jobs" collection, relaunch the jobs for the waiting or started datasets, then delete the "jobs" collection.