Improve the table job_stats

Another small simplification step. This pull request remove two fields from the table job_stats, i.e. num_tasks and num_realizations, that are misleading and can be inferred from the performance table anyway. It adds instead a disk_space field, that will contain the change in database size during the given calculation (of course this number will be reliable only if the calculation runs alone). A future pull request will populate the disk_space field.
See https://bugs.launchpad.net/oq-engine/+bug/1239529