Reactive operators to rewrite (tracking issue)

[akarnokd]

• Multi.dropWhile (Reimplement Multi.dropWhile + TCK test #1464)
• Multi.flatMapIterable (Reimplement Multi.flatMapIterable + TCK test #1467)
• Multi.collect (Rewrite collect, add juc.Collector overload + TCK tests #1459)
• Multi.first (Reimplement Multi.first + TCK test #1466)
• Multi.just(T...) (Reimplement Multi.just(T[]), add Multi.singleton(T) + TCK #1461)
• Single.map (Reimplement Single.map + TCK test #1456)
• Single.flatMap (Reimplement Single.flatMap(->Publisher) + TCK test #1465)
• Single.from(Publisher) (Reimplement Single.from + TCK tests #1481)
• Subscribable.onTerminate (Move onXXX from Subscribable to Single + TCK #1477)
• Subscribable.onComplete (Move onXXX from Subscribable to Single + TCK #1477)
• Subscribable.onError (Move onXXX from Subscribable to Single + TCK #1477)
• Subscribable.onErrorResume (Reimplement onErrorResume[With] + TCK tests #1489)
• Subscribable.onErrorResumeNext (Reimplement onErrorResume[With] + TCK tests #1489)

Remarks:

• These involve doing a Publisher implementation in place of a Processor implementation.
• Subscribables listed should be inlined into Multi and Single so they don't change the return type.
• Removing the former Processor implementation can become trouble if done one by one because of the microprofile-rs depending on them.

[akarnokd]

Closing as all related PRs have been merged.