2.x: Fix concurrent clear() calls when fused chains are canceled

[akarnokd]

Backport of #6676

When a fuseable source backed by an SpscLinkedArrayQueue is cancelled and cleared concurrently (i.e., one thread clears while the other cancels the chain), the clear() method could run concurrently and either crash with NPE or end up in an infinite loop due to corrupted queue state.

This PR fixes two kinds of mistakes leading to this scenario:

• Calling clear() from cancel/dispose when the output is fused.
• Calling clear() from a fused drain loop when cancellation is detected.

When fused, similar to poll(), calling clear() is the responsibility of the consumer and the producer side is not allowed to call them.

The bug affected the following operators:

• FlowableOnBackpressureBuffer
• FlowableGroupBy
• UnicastProcessor
• UnicastSubject

Fixes #6673

[codecov]

Codecov Report

Merging #6677 into 2.x will decrease coverage by 0.01%.
The diff coverage is 87.5%.

@@             Coverage Diff              @@
##                2.x    #6677      +/-   ##
============================================
- Coverage     98.19%   98.18%   -0.02%     
- Complexity     6347     6349       +2     
============================================
  Files           677      677              
  Lines         45551    45550       -1     
  Branches       6332     6333       +1     
============================================
- Hits          44731    44725       -6     
- Misses          256      260       +4     
- Partials        564      565       +1

Impacted Files	Coverage Δ	Complexity Δ
...x/internal/operators/flowable/FlowableGroupBy.java	96.06% <0%> (+0.54%)	3 <0> (ø)	⬇️
...ain/java/io/reactivex/subjects/UnicastSubject.java	100% <100%> (ø)	65 <0> (ø)	⬇️
...java/io/reactivex/processors/UnicastProcessor.java	100% <100%> (+1.19%)	68 <0> (+1)	⬆️
...erators/flowable/FlowableOnBackpressureBuffer.java	96.63% <100%> (ø)	2 <0> (ø)	⬇️
...ernal/operators/flowable/FlowableFlatMapMaybe.java	90.33% <0%> (-4.35%)	2% <0%> (ø)
...l/operators/observable/ObservableFlatMapMaybe.java	84.96% <0%> (-3.27%)	2% <0%> (ø)
...ava/io/reactivex/processors/BehaviorProcessor.java	96.86% <0%> (-2.25%)	60% <0%> (ø)
...internal/operators/flowable/FlowableSwitchMap.java	93.51% <0%> (-1.39%)	3% <0%> (ø)
...ex/internal/operators/flowable/FlowableCreate.java	96.77% <0%> (-0.97%)	6% <0%> (ø)
...ternal/operators/observable/ObservablePublish.java	99.12% <0%> (-0.88%)	11% <0%> (-1%)
... and 18 more

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