Preprocess up-to-date targets in mc_master

[kendonB]

I believe this will go a long way to solving the issue I was having in #449.

You probably could literally call outdated in this function, being careful to ensure outdated uses jobs_imports wherever it lightly parallelizes.

[wlandau]

It gets tricky because sometimes mc_master() is used to process the imports at the same time. But maybe we can remove up-to-date targets from the schedule here. That way, they will never get added to the priority queue.

[kendonB]

@wlandau If all targets are up to date, make_targets still runs run_parallel_backend unnecessarily. Since the worker launches but ends up doing nothing, this can still take a long time if the HPC scheduler doesn't launch your job right away. Would checking the length (however you do that for an igraph) of config$schedule do it?

[wlandau]

Sure. For distributed backends, it does make sense to check the schedule in make_targets().

[wlandau]

Fixed in de44794.