Rewrite the reactive saveAll(Publisher<T>) methods in template and repository #2496
Description
Currently the saveAll(Publisher) methods collect the publisher into a list, sending that to Elasticsearch as a batch request and then returning data from the response as Flux.
This needs to be changed to a more reactive approach, as it is not known how many items will be sent in with the publisher.
But we cannot simply just sent each element as a single index request, this would flood the network layer, so we need to
- buffer the incoming data into batches
- send this as a bulk request to Elasticsearch
- use backpressure handling with the upstream publisher to prevent flooding the transport client.
- provide the returned data in a continous flux.
I outlined the basic idea how this can be done in #2492