3.x: Fix MulticastProcessor not requesting more after limit is reached

[akarnokd]

After reaching the prefetch limit, the updated consumed counter was not written back so the processor stopped requesting more (consumed > limit).

Affects 2.x to. A separate PR will be posted for it.

Resolves #6713

[codecov]

Codecov Report

Merging #6714 into 3.x will decrease coverage by 0.07%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##                3.x    #6714      +/-   ##
============================================
- Coverage     98.16%   98.08%   -0.08%     
  Complexity     6191     6191              
============================================
  Files           677      677              
  Lines         44657    44658       +1     
  Branches       6170     6170              
============================================
- Hits          43839    43805      -34     
- Misses          293      315      +22     
- Partials        525      538      +13

Impacted Files	Coverage Δ	Complexity Δ
...activex/rxjava3/processors/MulticastProcessor.java	98.67% <100%> (ø)	84 <0> (ø)	⬇️
...a3/internal/operators/flowable/FlowableCreate.java	90.29% <0%> (-7.45%)	6% <0%> (ø)
...rnal/operators/flowable/FlowableFlatMapSingle.java	92.44% <0%> (-2.91%)	2% <0%> (ø)
...rnal/operators/observable/ObservableSwitchMap.java	91.57% <0%> (-2.64%)	3% <0%> (ø)
...rxjava3/internal/observers/QueueDrainObserver.java	97.43% <0%> (-2.57%)	21% <0%> (-1%)
.../operators/observable/ObservableFlatMapSingle.java	88.09% <0%> (-2.39%)	2% <0%> (ø)
...nternal/operators/observable/ObservableWindow.java	98% <0%> (-2%)	3% <0%> (ø)
...tivex/rxjava3/disposables/CompositeDisposable.java	98.14% <0%> (-1.86%)	39% <0%> (-1%)
...ava3/internal/operators/parallel/ParallelJoin.java	92.04% <0%> (-1.14%)	3% <0%> (ø)
...rxjava3/internal/schedulers/ExecutorScheduler.java	96% <0%> (-1%)	10% <0%> (ø)
... and 20 more

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