Parallel Gatherers

[pivovarit]

Since Gatherers are likely to be a thing next year - it's time to think how to leverage them.

The lack of such generic intermediate operation is the main driver that made me roll forward with the idea to push mapping into the collect phase. So, once that JEP is live, it would make sense to perform mapping inside gather, return results wrapped in single CompletableFuture and then use a dedicated Collector to combine them into a single CompletableFuture with a list of results