3.x: Standardize many of the operator fusion interfaces & queues

[akarnokd]

This PR opens up the internal fusion-related interfaces and the two most used atomic queue implementations to become standard:

• QueueFuseable
• QueueDisposable
• QueueSubscription
• ScalarSupplier
• SimpleQueue
• SimplePlainQueue
• SpscArrayQueue
• SpscLinkedArrayQueue

Their javadocs have been tidied up a bit as well.

Resolves #7318

[codecov]

Codecov Report

Merging #7320 (4e8e029) into 3.x (3e77f5c) will decrease coverage by 0.01%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##                3.x    #7320      +/-   ##
============================================
- Coverage     99.56%   99.55%   -0.02%     
+ Complexity     6784     6782       -2     
============================================
  Files           751      751              
  Lines         47490    47490              
  Branches       6378     6378              
============================================
- Hits          47284    47277       -7     
- Misses           95       96       +1     
- Partials        111      117       +6     

Impacted Files	Coverage Δ
.../main/java/io/reactivex/rxjava3/core/Flowable.java	100.00% <ø> (ø)
...ain/java/io/reactivex/rxjava3/core/Observable.java	100.00% <ø> (ø)
.../rxjava3/internal/disposables/EmptyDisposable.java	100.00% <ø> (ø)
.../internal/fuseable/AbstractEmptyQueueFuseable.java	90.90% <ø> (ø)
...a3/internal/fuseable/CancellableQueueFuseable.java	100.00% <ø> (ø)
...x/rxjava3/internal/jdk8/FlowableFlatMapStream.java	100.00% <ø> (ø)
...ivex/rxjava3/internal/jdk8/FlowableFromStream.java	100.00% <ø> (ø)
...vex/rxjava3/internal/jdk8/FlowableMapOptional.java	100.00% <ø> (ø)
...ex/rxjava3/internal/jdk8/ObservableFromStream.java	100.00% <ø> (ø)
...vex/rxjava3/internal/jdk8/ParallelMapOptional.java	100.00% <ø> (ø)
... and 129 more

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