3.x: Fix Flowable.groupBy cancellation/cleanup/eviction race hangs

[akarnokd]

This PR fixes three additional race conditions from #6974 (comment)

1. When a cancellation arrived asynchronously to a group, the drain loop quit and left unconsumed items in the queue and thus prevented the replenishment from the main upstream. Fix: change the drain loop to always clean & replenish upon seeing a canceled state.

2. When a cancellation was detected just after the drain loop has polled from the queue, that polled item was ignored and not replenished. Fix: Include the non-empty polled item in the replenishment count upon cancellation.

3. When a cancellation happened concurrently with an eviction, both could end up decrementing the group counter into an invalid (negative) number, preventing the operator to function properly. Fix: make sure group removal accounting happens at most once.

Fixes: #6974

[codecov]

Codecov Report

Merging #6979 into 3.x will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##                3.x    #6979   +/-   ##
=========================================
  Coverage     99.61%   99.61%           
  Complexity     6670     6670           
=========================================
  Files           742      742           
  Lines         47254    47270   +16     
  Branches       6371     6375    +4     
=========================================
+ Hits          47071    47088   +17     
  Misses           61       61           
+ Partials        122      121    -1     

Impacted Files	Coverage Δ	Complexity Δ
...3/internal/operators/flowable/FlowableGroupBy.java	99.70% <100.00%> (+0.32%)	3.00 <0.00> (ø)
...xjava3/internal/observers/FutureMultiObserver.java	96.61% <0.00%> (-3.39%)	27.00% <0.00%> (-1.00%)
...eactivex/rxjava3/processors/BehaviorProcessor.java	97.48% <0.00%> (-2.52%)	50.00% <0.00%> (ø%)
...3/internal/operators/flowable/FlowablePublish.java	98.01% <0.00%> (-1.99%)	17.00% <0.00%> (-1.00%)
...tivex/rxjava3/internal/jdk8/ParallelCollector.java	91.74% <0.00%> (-1.84%)	2.00% <0.00%> (ø%)
...nternal/operators/parallel/ParallelSortedJoin.java	98.49% <0.00%> (-1.51%)	2.00% <0.00%> (ø%)
...ternal/operators/completable/CompletableMerge.java	97.29% <0.00%> (-1.36%)	2.00% <0.00%> (ø%)
...rnal/operators/observable/ObservableObserveOn.java	98.69% <0.00%> (-1.31%)	3.00% <0.00%> (ø%)
...ternal/operators/observable/ObservableFlatMap.java	98.22% <0.00%> (-1.07%)	3.00% <0.00%> (ø%)
...a3/internal/operators/flowable/FlowableReplay.java	98.92% <0.00%> (-0.86%)	21.00% <0.00%> (ø%)
... and 12 more

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