- client: change work fetch policy to avoid starving GPUs in situations where GPU exclusions are used. - client: fix bug in round-robin simulation when GPU exclusions are used.

Note: this fixes a major problem (starvation)
    with project-level GPU exclusion.
    However, project-level GPU exclusion interferes with most of
    the client's scheduling policies.
    E.g., round-robin simulation doesn't take GPU exclusion into account,
    and the resulting completion estimates and device shortfalls
    can be wrong by an order of magnitude.

    The only way I can see to fix this would be to model each
    GPU instance as a separate resource,
    and to associate each job with a particular GPU instance.
    This would be a sweeping change in both client and server.