3.x: Disable fusion on the groups of Flowable.groupBy

[akarnokd]

This PR disables the async fusion capability of the groups emitted by Flowable.groupBy as it appears to lead to hangs due to cancellation and/or lack of requests in certain async scenarios.

By disabling fusion, the groups will manage the items they queue and cancellation will (hopefully) properly trigger replenishment requests for the unclaimed items.

This is more of a workaround than a comprehensible fix for the underlying issues. The main problem is that with groupBy and backpressure, each group itself and the items passing through them now count as resources and Reactive Streams can't cope well with items requiring their own lifecycle.

Related: #6982

[codecov]

Codecov Report

Merging #6983 into 3.x will decrease coverage by 0.05%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##                3.x    #6983      +/-   ##
============================================
- Coverage     99.61%   99.55%   -0.06%     
  Complexity     6669     6669              
============================================
  Files           742      742              
  Lines         47270    47267       -3     
  Branches       6375     6374       -1     
============================================
- Hits          47086    47055      -31     
- Misses           60       96      +36     
+ Partials        124      116       -8     

Impacted Files	Coverage Δ	Complexity Δ
...3/internal/operators/flowable/FlowableGroupBy.java	84.93% <ø> (-14.17%)	3.00 <0.00> (ø)
...tivex/rxjava3/disposables/CompositeDisposable.java	98.14% <0.00%> (-1.86%)	39.00% <0.00%> (-1.00%)
...nternal/operators/observable/ObservableCreate.java	97.43% <0.00%> (-1.71%)	2.00% <0.00%> (ø%)
...perators/observable/ObservableMergeWithSingle.java	99.05% <0.00%> (-0.95%)	2.00% <0.00%> (ø%)
...operators/observable/ObservableMergeWithMaybe.java	99.09% <0.00%> (-0.91%)	2.00% <0.00%> (ø%)
.../operators/observable/ObservableFlatMapSingle.java	96.82% <0.00%> (-0.80%)	2.00% <0.00%> (ø%)
...activex/rxjava3/processors/MulticastProcessor.java	99.54% <0.00%> (-0.46%)	82.00% <0.00%> (-1.00%)
...operators/flowable/FlowableConcatMapScheduler.java	99.60% <0.00%> (+0.39%)	4.00% <0.00%> (ø%)
.../operators/observable/ObservableCombineLatest.java	100.00% <0.00%> (+0.61%)	6.00% <0.00%> (ø%)
... and 6 more

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