Sync fusion support in reworked lazy mono stack

[OlegDokuka]

Sync fusion can be a nice improvement for the reworked lazy mono for scenarios where the source is static data source

follow up for #3081

[OlegDokuka]

Disposable d = Mono.fromFuture(future) // source is fn 
                                    .flatMap() // flat map can not fuse since the source is just push based observable
                                    .subscribe();
                                    // 5 sec wait
                                    d.dispose() // after some time

NO FUSION

subscriber call source.subscribe()
source call subscriber.onSubscribe()
subscriber call subscription.requestFusion()
subscriber call subscription.request()
subscription call subscriber.onNext()
subscription call subscriber.onComplete()

Disposable d = Mono.fromFuture(future)
                                    .cache() // can be represented as a queue
                                    .flatMap() // flat map fuses with the source and treat it as a queue
                                    .subscribe();
                                    // 5 sec wait
                                    d.dispose() // after some time

ASYNC FUSION (makes no sense, just brings redundancy)

subscriber call source.subscribe()
source call subscriber.onSubscribe()
subscriber call subscription.requestFusion()
subscriber call subscription.request()
subscription call subscriber.onNext(null)
subscriber call subscription.poll() // extra step here
subscription call subscriber.onComplete()

Disposable d = Mono.fromCallable(future) // source is fn 
                                    .flatMap() // flat map fuses with the source is callable
                                    .subscribe();
                                    // 5 sec wait
                                    d.dispose() // after some time

Callable macro fusion (a.k.a MONO SYNC FUSION) (means value is generated synchronously via function call or scalar)

subscriber call source.subscribe()
subscriber call source.call()