Investigate Uni.combine() stalling under stress

[jponge]

See quarkusio/quarkus#50736 where switching to Uni.join() fixes the issue.

This might be a problem with the combination operator relying on CompletableStage.

[xstefank]

CompletableFuture or CompletionStage :)

[jponge]

Either way 🤣

[jponge]

See smallrye/smallrye-health#663 for the impacted code path.

[jponge]

Note to self: look into uni.combine().all().unis(a, b, c, d).with((a, b, c, d) -> ...) and uni.combine().all().unis(list).with(...)

[ivu-mawi]

jponge pointed me here from #1981

This would explain a permanent hang in our apps which I've been tracking down for a while now 👀