Allow search create/update events to be scheduled in batches

[SanderFlooris]

Currently Statamic listens to the save events on entities, assets etc., and runs the sync instantly.

This works, but is really inefficient if you save a lot of items, as this will cause a lot of requests.

It would be great if Statamic had the option to delay those index update requests, and then send them all as a batch. The updateWithinIndexes function might be a good location to add this feature.