by Jacob Klein.
    The new policy is roughly as follows:
    - find the highest-priority project P that is allowed
        to fetch work for a resource below buf_min
    - Ask P for work for all resources R below buf_max
        for which it's allowed to fetch work,
        unless there's a higher-priority project allowed
        to request work for R.
    If we're going to do an RPC to P for reasons other than work fetch,
    the policy is:
    - for each resource R for which P is the highest-priority project
        allowed to fetch work, and R is below buf_max,
        request work for R.