Use "requestIdleCallback" for tile processing

Related to #1407

Some tasks are time-sliced over multiple frames to split up the work and keep the frame rate up:

• PriorityQueue
  • Tiling content downloading, parsing, preprocessing
  • Image overlay processing
• QueryManager
  • Time slice point settling per frame
• LRUCache
  • Used for unloading TilesRendererBase tiles
  • Used for unloading data in UnloadTilesPlugin

This work could be more quickly completed if we allow for it to run in an idle callback for a fixed amount of time. This would require adjusting PriorityQueue and LRUCache to be able to run for a fixed amount of time in addition to optionally iterating over a fixed amount of items (timing can possibly default to Infinity?) so we can take advantage of idle time. One caveat is that it's not supported in Safari:

requestIdleCallback( idle => {

  // run for the provided idle time
  const runTime = idle.timeRemaining();
  queue.tryRunJobs( runTime );

  // schedule idle callback again until work is complete?

} );

Perhaps a utility function can be written that can schedule frame work in addition to idle callback? This can't replace the rAF because we still need a guaranteed processing on the next frame.

Start by tracing the time-slicing entry points in PriorityQueue, LRUCache, and QueryManager, then read related issue #1407. Determine how idle callbacks could bound work while preserving guaranteed next-frame processing and Safari compatibility. Done means the listed workloads can use idle time without replacing the existing requestAnimationFrame path.