write job statistics to guest namespace on instance exit

[garlick]

Problem: we don't have much information about what happens in Flux instances launched by the system instance, unlike slurm which tracks job steps centrally.

Following up on a discussion in today's meeting about finding out what sort of "workflows" people are running: Could we have a flux instance write a summary object of some sort to the guest KVS directory in its enclosing instance?

A tool could summarize those objects at the system instance level.

For extra credit they could be made hierarchical, e.g. the object would include a summary of the objects written by sub-instances in their KVS.

FWIW, RFC 16 governs the content of the job KVS directory.