2.x: prepare the operator-fusion enhancement

[akarnokd]

This PR adds 3 operator-fusion related interfaces:

• QueueSubscription will support fusion for Flowable - avoid creating queues, avoid cost of multiple enqueue-dequeue, reduce request() overhead
• QueueDisposable will support fusion for Observable - avoid creating queues, avoid cost of multiple enqueue-dequeue
• ScalarCallable - indicate a scalar constant source

In addition ConditionalSubscriber is marked and will be moved to the new package and its method renamed to tryOnNext.

Unlike Reactor-Core (and Rsc), there is no Fuseable marker interface. The ability of fusing is indicated by calling onSubscribe with the above QueueX enabled instances and then called back via requestFusion. This allows having less duplicated code compared to Reactor/Rsc - at the cost of usually one extra field to store the QueueX value that may be null.

[codecov-io]

Current coverage is 70.09%

Merging #4103 into 2.x will decrease coverage by 0.01%

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- Hits          18919      18916     -3   
- Misses         6155       6159     +4   
+ Partials       1911       1910     -1   

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[artem-zinnatullin]

Nice 👍