Fixed bug adding new tasks to the pool.

On the iso branch, as a result of a bug in the runahead pool change, tasks with normal intercycle dependence that spawned beyond the suite stop point could be erroneously excluded from the task pool by the "future trigger beyond stop point" check (even though no future trigger exists here), instead of added to the pool in the held state.  Adding in the held state was what I had intended should happen, so that all tasks (except those whose recurrences end prior to the suite stop point!) are still present in the pool, potentially allowing the stop point to be changed before shut down.

@benfitzpatrick - is this what you were talking about here?: https://github.com/cylc/cylc/pull/997#issue-36559655:

> This also fixes a problem with the runahead pool change where tasks
beyond the stop point are no longer present in the state file on
stop-point-based suite shutdown. This stalls particular suites.