This gives you a way to simulate the effects of app_config.xml
- client: piggyback requests for resources even if we're backed off from them
- client: change resource backoff logic
    Old: if we requested work and didn't get any,
        back off from resources for which we requested work
    New: for each resource type T:
        if we requested work for T and didn't get any, back off from T
        Also, don't back off if we're already backed off
            (i.e. if this is a piggyback request)
        Also, only back off if the RPC was due to an automatic
            and potentially rapid source
            (namely: work fetch, result report, trickle up)
- client: fix small work fetch bug