3.x: Fix Flowable.window (size, time) cancellation and abandonment

[akarnokd]

This PR fixes the Flowable.window operator (with size and time boundaries) so that

• cancelling the inner windows allows cancelling the upstream once neither the main output nor other windows are being consumed further
• ignoring a window still allows cancelling the upstream.

These fixes are necessary similar to how groupBy had to be updated in #6740.

The publisher-boundary and start-stop publisher boundary as well as the Observable variants will be updated in subsequent PRs due to the sheer number of changes.

[codecov]

Codecov Report

Merging #6758 into 3.x will increase coverage by 0.06%.
The diff coverage is 93.99%.

@@             Coverage Diff              @@
##                3.x    #6758      +/-   ##
============================================
+ Coverage     98.08%   98.14%   +0.06%     
- Complexity     6190     6194       +4     
============================================
  Files           677      678       +1     
  Lines         44626    44580      -46     
  Branches       6161     6152       -9     
============================================
- Hits          43773    43755      -18     
+ Misses          310      295      -15     
+ Partials        543      530      -13

Impacted Files	Coverage Δ	Complexity Δ
.../main/java/io/reactivex/rxjava3/core/Flowable.java	100% <ø> (ø)	559 <0> (ø)	⬇️
...a3/internal/operators/flowable/FlowableWindow.java	89.8% <85.1%> (-1.73%)	4 <0> (ø)
...l/operators/flowable/WindowSubscribeIntercept.java	87.5% <87.5%> (ø)	4 <4> (?)
...ternal/operators/flowable/FlowableWindowTimed.java	96.63% <95.68%> (+3.84%)	5 <1> (+1)	⬆️
...l/operators/observable/ObservableFlatMapMaybe.java	83.8% <0%> (-4.93%)	2% <0%> (ø)
...rnal/operators/flowable/FlowableFlatMapSingle.java	92.44% <0%> (-2.91%)	2% <0%> (ø)
...perators/single/SingleFlatMapIterableFlowable.java	94.16% <0%> (-2.5%)	2% <0%> (ø)
...ava3/internal/operators/maybe/MaybeMergeArray.java	96.04% <0%> (-1.13%)	6% <0%> (ø)
.../operators/observable/ObservableCombineLatest.java	96.17% <0%> (-0.64%)	6% <0%> (ø)
...activex/rxjava3/processors/MulticastProcessor.java	98.23% <0%> (-0.45%)	83% <0%> (-1%)
... and 13 more

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[vanniktech]

👍