When a job takes a long time to execute but is scheduled often, it might be desirable to avoid concurrent executions of the job.

It seems like implementing the StatefulJob interface could be enough to achieve this. Here's a simple patch that allows Magnolia jobs to be configured as non-concurrent.

To review: implications of this change:

1. should we make this the default behavior ? (attached patch keeps the concurrent flag to true, i.e. the current default behavior)
2. do we use a store for our jobs, should we, especially if we're starting to have long-running jobs ?