Io Scheduler, Scheduled worker release.

[sgc-code]

IoScheduler releases the workers back to the pool to be reused optimistically to reduce the number of threads being created. This could lead to deadlocks in some situations like this one:

Flowable.just("item") .observeOn(Schedulers.io()) .firstOrError() .subscribe(__ -> { for (int i = 0; i < 200; i++) { Completable.complete() .observeOn(Schedulers.io()) .blockingAwait(); } System.out.println("Will never reach this"); }); 

As discussed on #7153, this PR allow to configure this behaviour.
Enabling rx3.io-scheduled-release will guarantee that workers are not reused until all the processing has finished.

Once we merge this we, I can create a similar PR for the 2.x branch

[codecov]

Codecov Report

Merging #7160 (526d600) into 3.x (e46ce76) will increase coverage by 0.03%.
The diff coverage is 100.00%.

@@ Coverage Diff @@ ## 3.x #7160 +/- ## ============================================ + Coverage 99.49% 99.53% +0.03%  Complexity 6708 6708 ============================================ Files 745 745 Lines 47327 47332 +5 Branches 6375 6376 +1 ============================================ + Hits 47089 47111 +22  + Misses 104 98 -6  + Partials 134 123 -11 

Impacted Files	Coverage Δ	Complexity Δ
...va/io/reactivex/rxjava3/schedulers/Schedulers.java	100.00% <ø> (ø)	12.00 <0.00> (ø)
...tivex/rxjava3/internal/schedulers/IoScheduler.java	97.91% <100.00%> (+0.11%)	9.00 <0.00> (ø)
...l/operators/observable/ObservableFlatMapMaybe.java	91.54% <0.00%> (-4.23%)	2.00% <0.00%> (ø%)
...java3/internal/operators/flowable/FlowableZip.java	98.97% <0.00%> (-1.03%)	6.00% <0.00%> (ø%)
.../operators/observable/ObservableCombineLatest.java	99.38% <0.00%> (-0.62%)	6.00% <0.00%> (ø%)
...a/io/reactivex/rxjava3/subjects/ReplaySubject.java	99.16% <0.00%> (+0.20%)	49.00% <0.00%> (ø%)
...ternal/operators/observable/ObservableGroupBy.java	100.00% <0.00%> (+0.54%)	2.00% <0.00%> (ø%)
...rnal/operators/flowable/FlowableFlatMapSingle.java	94.18% <0.00%> (+0.58%)	2.00% <0.00%> (ø%)
.../operators/observable/ObservableFlatMapSingle.java	96.03% <0.00%> (+0.79%)	2.00% <0.00%> (ø%)
.../operators/maybe/MaybeFlatMapIterableFlowable.java	100.00% <0.00%> (+0.81%)	2.00% <0.00%> (ø%)
... and 9 more

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