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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