ExtractSourcePackageFiles task needs to refresh its lock regularly

The ExtractSourcePackageFiles task often takes very long, way more than the 30 minutes allocated by the initial lock grant. Thus to really prevent concurrent runs, we need to refresh the lock regularly.

There is currently no code to refresh the lock. It should be added in TaskData where get_run_lock currently resides as well as some helper extend_run_lock in BaseTask.

In the same spirit, the task should regularly save its associated TaskData so that processed entries do not get reprocessed in case it gets interrupted before the end.