2.x: fix periodic scheduler purging config not honored

[akarnokd]

The SchedulerPoolFactory did not act properly on the system properties setup.

Reported on StackOverflow: https://stackoverflow.com/q/44717193/61158

[codecov]

Codecov Report

Merging #5441 into 2.x will increase coverage by 0.11%.
The diff coverage is 25%.

@@             Coverage Diff              @@
##                2.x    #5441      +/-   ##
============================================
+ Coverage     95.96%   96.08%   +0.11%     
- Complexity     5793     5804      +11     
============================================
  Files           631      631              
  Lines         41297    41301       +4     
  Branches       5744     5745       +1     
============================================
+ Hits          39632    39684      +52     
+ Misses          663      631      -32     
+ Partials       1002      986      -16

Impacted Files	Coverage Δ	Complexity Δ
...ivex/internal/schedulers/SchedulerPoolFactory.java	71.73% <25%> (-4.46%)	7 <4> (ø)
...vex/internal/operators/single/SingleTakeUntil.java	88.52% <0%> (-6.56%)	2% <0%> (ø)
...ernal/operators/maybe/MaybeTakeUntilPublisher.java	96% <0%> (-4%)	2% <0%> (ø)
...l/operators/observable/ObservableFlatMapMaybe.java	84.96% <0%> (-3.93%)	2% <0%> (ø)
...internal/operators/completable/CompletableAmb.java	94.91% <0%> (-3.39%)	10% <0%> (-1%)
...nternal/operators/observable/ObservableCreate.java	94.87% <0%> (-2.57%)	2% <0%> (ø)
...activex/internal/observers/QueueDrainObserver.java	61.53% <0%> (-2.57%)	12% <0%> (-1%)
...operators/observable/ObservableConcatMapEager.java	97.26% <0%> (-2.19%)	2% <0%> (ø)
...ernal/operators/flowable/FlowableFromIterable.java	93.04% <0%> (-2.14%)	5% <0%> (ø)
...rnal/operators/observable/ObservableObserveOn.java	97.95% <0%> (-2.05%)	3% <0%> (ø)
... and 43 more

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