advanced job dependencies

[SteVwonder]

Add support to both the interfaces and the scheduling logic for dependencies between jobs.

In terms of interfaces:

• when users submit a job, they will need a way to specify dependencies, which will probably require co-development with #1570 and potentially #1332
• command line interfaces for job dependency modification while the job is still in the pending queue

In terms of scheduling logic:

• We should look into how other schedulers handle job dependency logic
• We should perform performance testing based on the expected behavior of our users to ensure that long chains of dependencies do not place an undue burden on the scheduler

[dongahn]

We still have to decide where the dependency logic should sit between flux-core and flux-sched, though.

[SteVwonder]

Ah, good point. I just assumed it would go into sched, but you are right, that isn't necessarily the case.

[dongahn]

As we discussed this with Montoya, Dahlgren and Laney the other day, it seems we do need to collect input from some of the workflow people on this.