allow job duration of pending jobs to be updated

[garlick]

This is a tracking issue for allowing jobspec duration to be updated on a job after it is submitted, but before it is allocated resources.

Note that the extension of expiration time on running jobs is a separate issue. See #4175.

The duration is part of jobspec, thus changing it should employ the jobspec-update event. The change should only be permitted if requested by the instance owner or the job owner, and must respect the duration limit (general or for the selected queue) if configured. If an alloc request has already been sent to the scheduler, it needs to be canceled and resent with the new jobspec.

• need job manager service method for updating duration of pending job
• need front end tool for requesting duration update
• job-list should process the jobspec-update event so that the new duration can be returned to users job-list: handle jobspec-update event #4404

[grondo]

This is now complete.