3.x: Add fair mode overload to Schedulers.from(Executor)

[akarnokd]

The default Schedulers.from implementation uses an eager approach to execute tasks on the underlying Executor which can lead to excessive thread occupation on its own, even if operators use buffering/prefetch of 1.

This PR introduces a new overload with a fairness option so that tasks are submitted to the Executor in a non-overlapping and FIFO manner one by one.

In addition, the scheduler retention tests have been cleaned up and reworked to not wait unnecessarily long for the GC to finish.

Related: #6696, #6697

[codecov]

Codecov Report

Merging #6744 into 3.x will increase coverage by <.01%.
The diff coverage is 96%.

@@             Coverage Diff              @@
##                3.x    #6744      +/-   ##
============================================
+ Coverage     98.12%   98.13%   +<.01%     
  Complexity     6189     6189              
============================================
  Files           677      677              
  Lines         44663    44683      +20     
  Branches       6171     6176       +5     
============================================
+ Hits          43825    43849      +24     
- Misses          299      301       +2     
+ Partials        539      533       -6

Impacted Files	Coverage Δ	Complexity Δ
...va/io/reactivex/rxjava3/schedulers/Schedulers.java	100% <100%> (ø)	12 <3> (+1)	⬆️
...rxjava3/internal/schedulers/ExecutorScheduler.java	95.89% <95.45%> (-1.11%)	10 <2> (ø)
...perators/single/SingleFlatMapIterableFlowable.java	94.16% <0%> (-2.5%)	2% <0%> (ø)
...eactivex/rxjava3/processors/BehaviorProcessor.java	96.58% <0%> (-2.44%)	51% <0%> (ø)
...tivex/rxjava3/disposables/CompositeDisposable.java	98.14% <0%> (-1.86%)	39% <0%> (-1%)
...l/operators/observable/ObservableFlatMapMaybe.java	90.84% <0%> (-1.41%)	2% <0%> (ø)
...ternal/operators/completable/CompletableMerge.java	95.94% <0%> (-1.36%)	2% <0%> (ø)
...a3/internal/operators/flowable/FlowableCreate.java	96.76% <0%> (-1.3%)	6% <0%> (ø)
...nternal/operators/observable/ObservableCreate.java	97.43% <0%> (-0.86%)	2% <0%> (ø)
... and 13 more

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