Resources passed to generators (and maybe subdags?) should only depend on sub-workflow operators that use the resource

[mchalek]

We have seen behavior in which resources passed to generators containing some operators that use the resources, and other operators that do not use the resources, are not destroyed until every operator in the generator sub-workflow is complete. But this could mean keeping clusters around much longer than necessary, if the generator workflow contains long-running resource-independent operations.

This probably also applies to sub-dags, because resources are attached to them using the same mechanisms.