flux-accounting job-archive: implement purge of old jobs

[cmoussa1]

Broken off from a comment in #353, fetching new jobs periodically and adding it to the flux-accounting DB will grow the DB incredibly large. And since job-usage and fair share (by default) only consider jobs up to a certain point, perhaps a purge_old_jobs () function should be considered to be added (side note: I think flux-core is considering different methods of storing old jobs "forever" in some sort of historical storage, so it wouldn't be flux-accounting's responsibility to keep jobs forever at this moment).

flux-accounting currently keeps a timestamp of the t_inactive of the last completed job it saw when it fetches jobs from the flux-core job-archive DB as well as values for how long to consider jobs for job-usage, so it could probably make use of both of those (most likely just the latter) to decide when to purge jobs from its job-archive.

[cmoussa1]

This should now be fixed by #459 with the introduction of the scrub-old-jobs command. Can re-open more specific issues if ones arise.