Maximum number of sidecar videos awake at once, Infinity for no limit
Seconds of priority candidate has to be ahead of holder to take its slot, so slots don't flip between two candidates
Releases awake slot, waiting candidates are notified.
Acquires awake slot, returns false when candidate has to wait.
Removes candidate from holders and waiting candidates, once it doesn't need to be awake.
Limits number of sidecar videos awake at once, see SidecarVideoResidency.
When all slots are taken, candidate more important than least important holder by more than preemptMargin takes its slot, otherwise it waits and is notified once a slot is free.