hydra-queue-runner: Set the start time properly
hydra-queue-runner: Cache the lookup of time spent per jobset
hydra-queue-runner: Tweaked the selection method

Pick the jobset that has used the smallest fraction of its share,
rather than the jobset furthest below its share in absolute terms.
This gives jobsets with a small share a quicker start (but they
will also run out of their share quicker).