2.x: Fix groupBy not requesting more if a group is cancelled with buffered items

[akarnokd]

If a group is cancelled with unconsumed item in its buffer, the operator stopped requesting more thus hanging other groups.

The fix is to count how many items are still in the buffer upon cancellation, and issue a request for them for the parent.

Fixes #6889

[codecov]

Codecov Report

Merging #6894 into 2.x will increase coverage by 0.09%.
The diff coverage is 63.63%.

@@             Coverage Diff              @@
##                2.x    #6894      +/-   ##
============================================
+ Coverage     98.12%   98.21%   +0.09%     
- Complexity     6343     6348       +5     
============================================
  Files           677      677              
  Lines         45535    45541       +6     
  Branches       6336     6339       +3     
============================================
+ Hits          44680    44727      +47     
+ Misses          292      257      -35     
+ Partials        563      557       -6

Impacted Files	Coverage Δ	Complexity Δ
...x/internal/operators/flowable/FlowableGroupBy.java	95.36% <63.63%> (-0.76%)	3 <0> (ø)
...ernal/operators/flowable/FlowableFlatMapMaybe.java	90.82% <0%> (-2.9%)	2% <0%> (ø)
.../operators/observable/ObservableFlatMapSingle.java	88.8% <0%> (-2.24%)	2% <0%> (ø)
.../io/reactivex/disposables/CompositeDisposable.java	98.14% <0%> (-1.86%)	39% <0%> (-1%)
...ternal/operators/flowable/FlowableSubscribeOn.java	96.61% <0%> (-1.7%)	2% <0%> (ø)
...ava/io/reactivex/processors/BehaviorProcessor.java	96.86% <0%> (-1.35%)	60% <0%> (+1%)
...ernal/operators/flowable/FlowableFromIterable.java	95.18% <0%> (-1.07%)	5% <0%> (ø)
...perators/observable/ObservableMergeWithSingle.java	99.06% <0%> (-0.94%)	2% <0%> (ø)
...operators/observable/ObservableMergeWithMaybe.java	99.1% <0%> (-0.9%)	2% <0%> (ø)
...perators/single/SingleFlatMapIterableFlowable.java	97.5% <0%> (-0.84%)	2% <0%> (ø)
... and 17 more

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