Wire up the rest of the PrologFlags=SendGIDs support.

In slurmctld:
  - Handle the lookup, and send results across.
  - Periodically flush the cache.

In slurmd:
  - Load value sent as part of prolog into cache
  - Have task launch use the new call instead to fetch
    the jobid-cached values when available
  - Remove old job-specific records from the cache as
    part of the prolog.
  - Ignore the slowly increasing memory footprint from the cache,
    as this is what the old implementation did anyways. It'll be
    O(users in cluster) at worst, and flushes on a reconfigure.
  - Simplify the slurmstepd communication path. group_cache_lookup
    will always succeed.

Left to do:
  - Convert other lookup locations in slurmd to the new approach,
    and remove the compatibility shim.

Bug 3322.