Implement max_pending_time for the PBS scheduler

[ekouts]

[vkarak]

This is not trivial to implement as of now for the PBS scheduler, because we don't use the qstat command there, so we don't know while polling if a job is actually running of pending. It would be meaningful only if use qstat for the PBS backend as well, in which case, the Torque/PBS schedulers must be refactored: either the poll() of Torque be moved in PBS or the make the PBS scheduler inherit from Torque.