Variable runahead limit, for start-up graphs.

[hjoliver]

Supercedes #4903 (via an @oliver-sanders suggestion there).

Optionally tie the runahead limit down to P1 until the first cycle has finished, to avoid the ugliness of making tasks in every subsequent cycle depend explicitly on the first (if they aren't otherwise constrained by dependencies).

[oliver-sanders]

If possible might be worth trying to do the same for the final cycle point.