Fixed polling of orphaned tasks on restart.

The restart command had not been updated for very recent polling
developments; and task poll timers have to be created before job
submission so they can still be used for orphaned tasks resurrected
on suite restart. Closes #524.
Some comments.
Handle late 'submission succeeded' messages properly.

The started message can arrive first if a task starts running very quickly.
Merge pull request #528 from hjoliver/524.fix-polling-on-restart

Fixed polling of orphaned tasks on restart.