[idea] provide hostname to nodeset lookup in KVS

[garlick]

Is there a use case for being able to look up the flux session ranks that are running on a given hostname, e.g. the reverse of #892?

We disabled the population of resource-hwloc by_host by default in #716 for performance reasons that may have since been addressed by #732. One possibility is to re-enable that and add a nodeset key under it.

In order to write a nodeset, each rank would need to know its "clique ranks" which the broker already knows from its PMI bootstrap and could export in a broker attribute. With that information we could also have only the lowest numbered rank in the clique write the data.

[grondo]

This seems like a good idea, and I think there was a use case in flux-sched that @dongahn might remember.

[grondo]

This was partially implemented as part of #1499, to be removed again in #1988. Possibly no valid use case at this point? Reopen if so.