Discard tasks that have passed their own stop cycle.

They were being adding to task pool but in the 'held' state, which
could hold up the suite by means of the runahead limit.
Added a test for the insert command (with and without a stop cycle).
Merge pull request #879 from hjoliver/discard-tasks-beyond-their-stop-cycle

Discard tasks that have passed their own stop cycle.