Add poll_now_notify parameter to poll_loop and on_work_available callback

[phillipleblanc]

• Add poll_now_notify: Option<Arc> parameter to poll_loop in execution_loop.rs
• Update idle sleep to use tokio::select! - wakes on 100ms timeout OR notify signal
• Add OnWorkAvailableFn callback type and field to SchedulerConfig
• Invoke callback after JobSubmitted and TaskUpdating events when new work available
• Update standalone executor and executor_process to pass None for poll_now_notify

[milenkovicm]

Hey @phillipleblanc I've noticed you guys are doing great job extending ballista, would you consider contributing patches back to ballista for things like #12, #13, #14, #15 ?

[milenkovicm]

Also, it would be great if could get involved into steering ballista's direction

[phillipleblanc]

@milenkovicm Yes! We can definitely upstream some of the work we've been doing. What would be the best way to start participating in steering ballista's direction?

[milenkovicm]

cool! if ok, i would cherry pick few commits from your repo an back-port them to ballista, will add you do PR review. also, feel free to suggest things needed on master

with regards to ballista steering participations, whatever you guys have bandwidth to do, even just opening bugs or keeping an eye on PR would be appreciated